feat: Enhance journal entry and transaction handling with posting keys

- Added posting key support to SKR03 and SKR04 journal entries and transactions to ensure DATEV compliance.
- Implemented validation for posting keys in journal entries, ensuring all lines have a posting key and that they are consistent across the entry.
- Introduced automatic account checks to prevent posting to accounts that cannot be directly posted to (e.g., 1400, 1600).
- Updated account validation to include checks for debtor and creditor ranges.
- Enhanced invoice booking logic to include appropriate posting keys based on VAT rates and scenarios.
- Created a new module for posting key definitions and validation rules, including functions for validating posting keys and suggesting appropriate keys based on transaction parameters.
- Updated tests to cover new posting key functionality and ensure compliance with accounting rules.

test/test.jahresabschluss.skr03.ts

@@ -17,6 +17,24 @@ tap.test('should demonstrate complete Jahresabschluss (Annual Financial Statemen
 
   await api.initialize('SKR03');
   expect(api.getSKRType()).toEqual('SKR03');
+
+  // Create debtor account (customer) - replaces automatic account 1400
+  await api.createAccount({
+    accountNumber: '10001',
+    accountName: 'Kunde Mustermann GmbH',
+    accountClass: 1,
+    accountType: 'asset',
+    skrType: 'SKR03',
+  });
+
+  // Create creditor account (supplier) - replaces automatic account 1600
+  await api.createAccount({
+    accountNumber: '70001',
+    accountName: 'Lieferant Test GmbH',
+    accountClass: 7,
+    accountType: 'liability',
+    skrType: 'SKR03',
+  });
 });
 
 tap.test('should set up opening balances (Eröffnungsbilanz)', async () => {
@@ -24,6 +42,9 @@ tap.test('should set up opening balances (Eröffnungsbilanz)', async () => {
   // This represents a small GmbH (limited liability company)
   // Using only accounts that exist in SKR03
 
+  // Note: Opening balance entries use posting key 40 (tax-free) as they are internal closing entries
+  // Using personal accounts (10001 for debtor, 70001 for creditor) instead of automatic accounts
+
   // Post opening journal entry (Eröffnungsbuchung)
   const openingEntry = await api.postJournalEntry({
     date: new Date('2024-01-01'),
@@ -31,20 +52,20 @@ tap.test('should set up opening balances (Eröffnungsbilanz)', async () => {
     reference: 'EB-2024',
     lines: [
       // Debit all asset accounts
-      { accountNumber: '0200', debit: 45000, description: 'Grundstücke' },
-      { accountNumber: '0210', debit: 120000, description: 'Gebäude' },
-      { accountNumber: '0500', debit: 35000, description: 'Betriebs- und Geschäftsausstattung' },
-      { accountNumber: '0400', debit: 8000, description: 'Fuhrpark' },
-      { accountNumber: '1200', debit: 25000, description: 'Bank' },
-      { accountNumber: '1000', debit: 2500, description: 'Kasse' },
-      { accountNumber: '1400', debit: 18000, description: 'Forderungen' },
-      { accountNumber: '3100', debit: 12000, description: 'Warenvorräte' },
+      { accountNumber: '0200', debit: 45000, description: 'Grundstücke', postingKey: 40 },
+      { accountNumber: '0210', debit: 120000, description: 'Gebäude', postingKey: 40 },
+      { accountNumber: '0500', debit: 35000, description: 'Betriebs- und Geschäftsausstattung', postingKey: 40 },
+      { accountNumber: '0400', debit: 8000, description: 'Fuhrpark', postingKey: 40 },
+      { accountNumber: '1200', debit: 25000, description: 'Bank', postingKey: 40 },
+      { accountNumber: '1000', debit: 2500, description: 'Kasse', postingKey: 40 },
+      { accountNumber: '10001', debit: 18000, description: 'Forderungen Kunde', postingKey: 40 },
+      { accountNumber: '3100', debit: 12000, description: 'Warenvorräte', postingKey: 40 },
 
       // Credit all liability and equity accounts
-      { accountNumber: '2000', credit: 150000, description: 'Eigenkapital' },
-      { accountNumber: '2900', credit: 35000, description: 'Gewinnrücklagen' },
-      { accountNumber: '1600', credit: 52500, description: 'Verbindlichkeiten L+L' },
-      { accountNumber: '3300', credit: 28000, description: 'Verbindlichkeiten Kreditinstitute' },
+      { accountNumber: '2000', credit: 150000, description: 'Eigenkapital', postingKey: 40 },
+      { accountNumber: '2900', credit: 35000, description: 'Gewinnrücklagen', postingKey: 40 },
+      { accountNumber: '70001', credit: 52500, description: 'Verbindlichkeiten Lieferant', postingKey: 40 },
+      { accountNumber: '3300', credit: 28000, description: 'Verbindlichkeiten Kreditinstitute', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -57,28 +78,28 @@ tap.test('should set up opening balances (Eröffnungsbilanz)', async () => {
 tap.test('should record Q1 business transactions', async () => {
   // January - March transactions
   
-  // Sale of goods with 19% VAT
+  // Sale of goods with 19% VAT - using debtor account 10001 instead of automatic 1400
   await api.postJournalEntry({
     date: new Date('2024-01-15'),
     description: 'Verkauf Waren auf Rechnung',
     reference: 'RE-2024-001',
     lines: [
-      { accountNumber: '1400', debit: 11900, description: 'Forderungen inkl. USt' },
-      { accountNumber: '8400', credit: 10000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 1900, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 11900, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '8400', credit: 10000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 1900, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Purchase of materials with 19% VAT
+  // Purchase of materials with 19% VAT - using creditor account 70001 instead of automatic 1600
   await api.postJournalEntry({
     date: new Date('2024-01-20'),
     description: 'Einkauf Material auf Rechnung',
     reference: 'ER-2024-001',
     lines: [
-      { accountNumber: '5400', debit: 5000, description: 'Wareneingang 19% Vorsteuer' },
-      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%' },
-      { accountNumber: '1600', credit: 5950, description: 'Verbindlichkeiten' },
+      { accountNumber: '5400', debit: 5000, description: 'Wareneingang 19% Vorsteuer', postingKey: 40 },
+      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '70001', credit: 5950, description: 'Verbindlichkeiten', postingKey: 9 },
     ],
     skrType: 'SKR03',
   });
@@ -89,21 +110,21 @@ tap.test('should record Q1 business transactions', async () => {
     description: 'Gehaltszahlung Januar',
     reference: 'GH-2024-01',
     lines: [
-      { accountNumber: '6000', debit: 8000, description: 'Löhne und Gehälter' },
-      { accountNumber: '6100', debit: 1600, description: 'Sozialversicherung AG-Anteil' },
-      { accountNumber: '1200', credit: 9600, description: 'Banküberweisung' },
+      { accountNumber: '6000', debit: 8000, description: 'Löhne und Gehälter', postingKey: 40 },
+      { accountNumber: '6100', debit: 1600, description: 'Sozialversicherung AG-Anteil', postingKey: 40 },
+      { accountNumber: '1200', credit: 9600, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Customer payment received
+  // Customer payment received - using debtor account 10001 instead of automatic 1400
   await api.postJournalEntry({
     date: new Date('2024-02-10'),
     description: 'Zahlungseingang Kunde',
     reference: 'ZE-2024-001',
     lines: [
-      { accountNumber: '1200', debit: 11900, description: 'Bankgutschrift' },
-      { accountNumber: '1400', credit: 11900, description: 'Forderungsausgleich' },
+      { accountNumber: '1200', debit: 11900, description: 'Bankgutschrift', postingKey: 40 },
+      { accountNumber: '10001', credit: 11900, description: 'Forderungsausgleich', postingKey: 3 },
     ],
     skrType: 'SKR03',
   });
@@ -114,8 +135,8 @@ tap.test('should record Q1 business transactions', async () => {
     description: 'Miete Februar',
     reference: 'MI-2024-02',
     lines: [
-      { accountNumber: '7100', debit: 2000, description: 'Miete' },
-      { accountNumber: '1200', credit: 2000, description: 'Banküberweisung' },
+      { accountNumber: '7100', debit: 2000, description: 'Miete', postingKey: 40 },
+      { accountNumber: '1200', credit: 2000, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -126,9 +147,9 @@ tap.test('should record Q1 business transactions', async () => {
     description: 'Büromaterial',
     reference: 'BM-2024-001',
     lines: [
-      { accountNumber: '6800', debit: 200, description: 'Bürobedarf' },
-      { accountNumber: '1571', debit: 38, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 238, description: 'Bankzahlung' },
+      { accountNumber: '6800', debit: 200, description: 'Bürobedarf', postingKey: 40 },
+      { accountNumber: '1571', debit: 38, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 238, description: 'Bankzahlung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -139,22 +160,22 @@ tap.test('should record Q1 business transactions', async () => {
     description: 'Tankrechnung Firmenfahrzeug',
     reference: 'KFZ-2024-001',
     lines: [
-      { accountNumber: '7400', debit: 150, description: 'Kfz-Kosten' },
-      { accountNumber: '1571', debit: 28.50, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 178.50, description: 'Bankzahlung' },
+      { accountNumber: '7400', debit: 150, description: 'Kfz-Kosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 28.50, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 178.50, description: 'Bankzahlung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Another sale
+  // Another sale - using debtor account 10001 instead of automatic 1400
   await api.postJournalEntry({
     date: new Date('2024-03-20'),
     description: 'Verkauf Dienstleistung',
     reference: 'RE-2024-002',
     lines: [
-      { accountNumber: '1400', debit: 7140, description: 'Forderungen inkl. USt' },
-      { accountNumber: '8400', credit: 6000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 1140, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 7140, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '8400', credit: 6000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 1140, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -169,35 +190,35 @@ tap.test('should record Q2-Q4 business transactions', async () => {
     description: 'Kauf neue Produktionsmaschine',
     reference: 'INV-2024-001',
     lines: [
-      { accountNumber: '0500', debit: 25000, description: 'Neue Maschine' },
-      { accountNumber: '1571', debit: 4750, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 29750, description: 'Banküberweisung' },
+      { accountNumber: '0500', debit: 25000, description: 'Neue Maschine', postingKey: 40 },
+      { accountNumber: '1571', debit: 4750, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 29750, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Q2: Large sale
+  // Q2: Large sale - using debtor account 10001
   await api.postJournalEntry({
     date: new Date('2024-05-10'),
     description: 'Großauftrag Kunde ABC',
     reference: 'RE-2024-003',
     lines: [
-      { accountNumber: '1400', debit: 35700, description: 'Forderungen inkl. USt' },
-      { accountNumber: '8400', credit: 30000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 5700, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 35700, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '8400', credit: 30000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 5700, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Q3: Marketing expenses
+  // Q3: Marketing expenses - using creditor account 70001
   await api.postJournalEntry({
     date: new Date('2024-07-10'),
     description: 'Werbekampagne',
     reference: 'WK-2024-001',
     lines: [
-      { accountNumber: '6600', debit: 5000, description: 'Werbekosten' },
-      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%' },
-      { accountNumber: '1600', credit: 5950, description: 'Verbindlichkeiten' },
+      { accountNumber: '6600', debit: 5000, description: 'Werbekosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '70001', credit: 5950, description: 'Verbindlichkeiten', postingKey: 9 },
     ],
     skrType: 'SKR03',
   });
@@ -208,9 +229,9 @@ tap.test('should record Q2-Q4 business transactions', async () => {
     description: 'Steuerberatung',
     reference: 'STB-2024-001',
     lines: [
-      { accountNumber: '6700', debit: 2500, description: 'Steuerberatungskosten' },
-      { accountNumber: '1571', debit: 475, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 2975, description: 'Banküberweisung' },
+      { accountNumber: '6700', debit: 2500, description: 'Steuerberatungskosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 475, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 2975, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -221,35 +242,35 @@ tap.test('should record Q2-Q4 business transactions', async () => {
     description: 'Jahresbonus Mitarbeiter',
     reference: 'BON-2024',
     lines: [
-      { accountNumber: '6000', debit: 10000, description: 'Tantieme' },
-      { accountNumber: '6100', debit: 2000, description: 'Sozialversicherung AG-Anteil' },
-      { accountNumber: '1200', credit: 12000, description: 'Banküberweisung' },
+      { accountNumber: '6000', debit: 10000, description: 'Tantieme', postingKey: 40 },
+      { accountNumber: '6100', debit: 2000, description: 'Sozialversicherung AG-Anteil', postingKey: 40 },
+      { accountNumber: '1200', credit: 12000, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Q4: Collection of outstanding receivables
+  // Q4: Collection of outstanding receivables - using debtor account 10001
   await api.postJournalEntry({
     date: new Date('2024-12-15'),
     description: 'Zahlungseingang Großauftrag',
     reference: 'ZE-2024-003',
     lines: [
-      { accountNumber: '1200', debit: 35700, description: 'Bankgutschrift' },
-      { accountNumber: '1400', credit: 35700, description: 'Forderungsausgleich' },
+      { accountNumber: '1200', debit: 35700, description: 'Bankgutschrift', postingKey: 40 },
+      { accountNumber: '10001', credit: 35700, description: 'Forderungsausgleich', postingKey: 3 },
     ],
     skrType: 'SKR03',
   });
 });
 
 tap.test('should perform year-end adjustments (Jahresabschlussbuchungen)', async () => {
-  // 1. Depreciation (Abschreibungen)
+  // 1. Depreciation (Abschreibungen) - internal adjustments use posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Abschreibung Gebäude (linear 2%)',
     reference: 'AFA-2024-001',
     lines: [
-      { accountNumber: '7000', debit: 2400, description: 'AfA auf Gebäude' },
-      { accountNumber: '0210', credit: 2400, description: 'Wertberichtigung Gebäude' },
+      { accountNumber: '7000', debit: 2400, description: 'AfA auf Gebäude', postingKey: 40 },
+      { accountNumber: '0210', credit: 2400, description: 'Wertberichtigung Gebäude', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -259,8 +280,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen)', async
     description: 'Abschreibung BGA (linear 10%)',
     reference: 'AFA-2024-002',
     lines: [
-      { accountNumber: '7000', debit: 6000, description: 'AfA auf BGA' }, // (35000 + 25000) * 10%
-      { accountNumber: '0500', credit: 6000, description: 'Wertberichtigung BGA' },
+      { accountNumber: '7000', debit: 6000, description: 'AfA auf BGA', postingKey: 40 }, // (35000 + 25000) * 10%
+      { accountNumber: '0500', credit: 6000, description: 'Wertberichtigung BGA', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -270,57 +291,57 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen)', async
     description: 'Abschreibung Fuhrpark (linear 20%)',
     reference: 'AFA-2024-003',
     lines: [
-      { accountNumber: '7000', debit: 1600, description: 'AfA auf Fuhrpark' },
-      { accountNumber: '0400', credit: 1600, description: 'Wertberichtigung Fuhrpark' },
+      { accountNumber: '7000', debit: 1600, description: 'AfA auf Fuhrpark', postingKey: 40 },
+      { accountNumber: '0400', credit: 1600, description: 'Wertberichtigung Fuhrpark', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // 2. Accruals (Rechnungsabgrenzung)
+  // 2. Accruals (Rechnungsabgrenzung) - internal adjustments use posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Aktive Rechnungsabgrenzung - Vorausbezahlte Versicherung',
     reference: 'ARA-2024-001',
     lines: [
-      { accountNumber: '1900', debit: 1000, description: 'Aktive Rechnungsabgrenzung' },
-      { accountNumber: '7300', credit: 1000, description: 'Versicherungen' },
+      { accountNumber: '1900', debit: 1000, description: 'Aktive Rechnungsabgrenzung', postingKey: 40 },
+      { accountNumber: '7300', credit: 1000, description: 'Versicherungen', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // 3. Provisions (Rückstellungen)
+  // 3. Provisions (Rückstellungen) - internal adjustments use posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Rückstellung für Jahresabschlusskosten',
     reference: 'RS-2024-001',
     lines: [
-      { accountNumber: '6700', debit: 3000, description: 'Rechts- und Beratungskosten' },
-      { accountNumber: '3000', credit: 3000, description: 'Rückstellungen' },
+      { accountNumber: '6700', debit: 3000, description: 'Rechts- und Beratungskosten', postingKey: 40 },
+      { accountNumber: '3000', credit: 3000, description: 'Rückstellungen', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // 4. Inventory adjustment
+  // 4. Inventory adjustment - internal adjustments use posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Bestandsveränderung Waren',
     reference: 'BV-2024-001',
     lines: [
-      { accountNumber: '3100', debit: 3000, description: 'Warenbestand Zugang' },
-      { accountNumber: '5900', credit: 3000, description: 'Bestandsveränderungen' },
+      { accountNumber: '3100', debit: 3000, description: 'Warenbestand Zugang', postingKey: 40 },
+      { accountNumber: '5900', credit: 3000, description: 'Bestandsveränderungen', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // 5. VAT clearing (Umsatzsteuer-Vorauszahlung)
+  // 5. VAT clearing (Umsatzsteuer-Vorauszahlung) - internal adjustments use posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'USt-Abschluss Q4',
     reference: 'UST-2024-Q4',
     lines: [
-      { accountNumber: '1771', debit: 8740, description: 'USt-Saldo' }, // Total collected VAT
-      { accountNumber: '1571', credit: 7191.50, description: 'Vorsteuer-Saldo' }, // Total input VAT
-      { accountNumber: '1800', credit: 1548.50, description: 'USt-Zahllast' },
+      { accountNumber: '1771', debit: 8740, description: 'USt-Saldo', postingKey: 40 }, // Total collected VAT
+      { accountNumber: '1571', credit: 7191.50, description: 'Vorsteuer-Saldo', postingKey: 40 }, // Total input VAT
+      { accountNumber: '1800', credit: 1548.50, description: 'USt-Zahllast', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
@@ -366,41 +387,41 @@ tap.test('should calculate income statement (GuV) before closing', async () => {
 tap.test('should perform closing entries (Abschlussbuchungen)', async () => {
   // Close all income and expense accounts to the profit/loss account
   
-  // Close revenue accounts
+  // Close revenue accounts - year-end closing uses posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Abschluss Ertragskonten',
     reference: 'AB-2024-001',
     lines: [
-      { accountNumber: '8400', debit: 46000, description: 'Erlöse abschließen' },
-      { accountNumber: '9400', credit: 46000, description: 'GuV-Konto' },
+      { accountNumber: '8400', debit: 46000, description: 'Erlöse abschließen', postingKey: 40 },
+      { accountNumber: '9400', credit: 46000, description: 'GuV-Konto', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Close expense accounts
+  // Close expense accounts - year-end closing uses posting key 40
   await api.postJournalEntry({
     date: new Date('2024-12-31'),
     description: 'Abschluss Aufwandskonten',
     reference: 'AB-2024-002',
     lines: [
-      { accountNumber: '9400', debit: 45450, description: 'GuV-Konto' },
-      { accountNumber: '7300', debit: 1000, description: 'Versicherung abschließen (credit balance)' },
-      { accountNumber: '5900', debit: 3000, description: 'Bestandsveränderungen abschließen (credit balance)' },
-      { accountNumber: '5400', credit: 5000, description: 'Wareneingang abschließen' },
-      { accountNumber: '6000', credit: 18000, description: 'Löhne und Gehälter abschließen' },
-      { accountNumber: '6100', credit: 3600, description: 'SV AG-Anteil abschließen' },
-      { accountNumber: '7000', credit: 10000, description: 'AfA abschließen' },
-      { accountNumber: '7100', credit: 2000, description: 'Miete abschließen' },
-      { accountNumber: '7400', credit: 150, description: 'Kfz abschließen' },
-      { accountNumber: '6600', credit: 5000, description: 'Werbung abschließen' },
-      { accountNumber: '6700', credit: 5500, description: 'Beratung abschließen' },
-      { accountNumber: '6800', credit: 200, description: 'Bürobedarf abschließen' },
+      { accountNumber: '9400', debit: 45450, description: 'GuV-Konto', postingKey: 40 },
+      { accountNumber: '7300', debit: 1000, description: 'Versicherung abschließen (credit balance)', postingKey: 40 },
+      { accountNumber: '5900', debit: 3000, description: 'Bestandsveränderungen abschließen (credit balance)', postingKey: 40 },
+      { accountNumber: '5400', credit: 5000, description: 'Wareneingang abschließen', postingKey: 40 },
+      { accountNumber: '6000', credit: 18000, description: 'Löhne und Gehälter abschließen', postingKey: 40 },
+      { accountNumber: '6100', credit: 3600, description: 'SV AG-Anteil abschließen', postingKey: 40 },
+      { accountNumber: '7000', credit: 10000, description: 'AfA abschließen', postingKey: 40 },
+      { accountNumber: '7100', credit: 2000, description: 'Miete abschließen', postingKey: 40 },
+      { accountNumber: '7400', credit: 150, description: 'Kfz abschließen', postingKey: 40 },
+      { accountNumber: '6600', credit: 5000, description: 'Werbung abschließen', postingKey: 40 },
+      { accountNumber: '6700', credit: 5500, description: 'Beratung abschließen', postingKey: 40 },
+      { accountNumber: '6800', credit: 200, description: 'Bürobedarf abschließen', postingKey: 40 },
     ],
     skrType: 'SKR03',
   });
 
-  // Transfer profit/loss to equity
+  // Transfer profit/loss to equity - year-end closing uses posting key 40
   const guv_result = 46000 - 45450; // Profit of 550
   if (guv_result > 0) {
     await api.postJournalEntry({
@@ -408,8 +429,8 @@ tap.test('should perform closing entries (Abschlussbuchungen)', async () => {
       description: 'Jahresgewinn auf Eigenkapital',
       reference: 'AB-2024-003',
       lines: [
-        { accountNumber: '9400', debit: guv_result, description: 'GuV-Konto ausgleichen' },
-        { accountNumber: '2900', credit: guv_result, description: 'Gewinnrücklagen' },
+        { accountNumber: '9400', debit: guv_result, description: 'GuV-Konto ausgleichen', postingKey: 40 },
+        { accountNumber: '2900', credit: guv_result, description: 'Gewinnrücklagen', postingKey: 40 },
       ],
       skrType: 'SKR03',
     });
@@ -419,8 +440,8 @@ tap.test('should perform closing entries (Abschlussbuchungen)', async () => {
       description: 'Jahresverlust auf Eigenkapital',
       reference: 'AB-2024-003',
       lines: [
-        { accountNumber: '2500', debit: Math.abs(guv_result), description: 'Verlustvortrag' },
-        { accountNumber: '9400', credit: Math.abs(guv_result), description: 'GuV-Konto ausgleichen' },
+        { accountNumber: '2500', debit: Math.abs(guv_result), description: 'Verlustvortrag', postingKey: 40 },
+        { accountNumber: '9400', credit: Math.abs(guv_result), description: 'GuV-Konto ausgleichen', postingKey: 40 },
       ],
       skrType: 'SKR03',
     });

test/test.jahresabschluss.skr04.ts

@@ -17,11 +17,30 @@ tap.test('should demonstrate complete Jahresabschluss (Annual Financial Statemen
 
   await api.initialize('SKR04');
   expect(api.getSKRType()).toEqual('SKR04');
+
+  // Create debtor account (customer) - replaces automatic account 1400
+  await api.createAccount({
+    accountNumber: '10001',
+    accountName: 'Kunde Mustermann GmbH',
+    accountClass: 1,
+    accountType: 'asset',
+    skrType: 'SKR04',
+  });
+
+  // Create creditor account (supplier) - replaces automatic account 1600
+  await api.createAccount({
+    accountNumber: '70001',
+    accountName: 'Lieferant Test GmbH',
+    accountClass: 7,
+    accountType: 'liability',
+    skrType: 'SKR04',
+  });
 });
 
 tap.test('should set up opening balances (Eröffnungsbilanz) for SKR04', async () => {
   // Opening balances from previous year's closing
   // SKR04 uses different account structure than SKR03
+  // Using personal accounts (10001 for debtor, 70001 for creditor) instead of automatic accounts
 
   // Post opening journal entry (Eröffnungsbuchung)
   const openingEntry = await api.postJournalEntry({
@@ -30,19 +49,19 @@ tap.test('should set up opening balances (Eröffnungsbilanz) for SKR04', async (
     reference: 'EB-2024',
     lines: [
       // Debit all asset accounts
-      { accountNumber: '0200', debit: 45000, description: 'Grundstücke' },
-      { accountNumber: '0210', debit: 120000, description: 'Gebäude' },
-      { accountNumber: '0500', debit: 35000, description: 'BGA' },
-      { accountNumber: '0400', debit: 8000, description: 'Fuhrpark' },
-      { accountNumber: '1200', debit: 25000, description: 'Bank' },
-      { accountNumber: '1000', debit: 2500, description: 'Kasse' },
-      { accountNumber: '1400', debit: 18000, description: 'Forderungen' },
+      { accountNumber: '0200', debit: 45000, description: 'Grundstücke', postingKey: 40 },
+      { accountNumber: '0210', debit: 120000, description: 'Gebäude', postingKey: 40 },
+      { accountNumber: '0500', debit: 35000, description: 'BGA', postingKey: 40 },
+      { accountNumber: '0400', debit: 8000, description: 'Fuhrpark', postingKey: 40 },
+      { accountNumber: '1200', debit: 25000, description: 'Bank', postingKey: 40 },
+      { accountNumber: '1000', debit: 2500, description: 'Kasse', postingKey: 40 },
+      { accountNumber: '10001', debit: 18000, description: 'Forderungen Kunde', postingKey: 40 },
 
       // Credit all liability and equity accounts
-      { accountNumber: '9000', credit: 150000, description: 'Eigenkapital' },
-      { accountNumber: '9300', credit: 35000, description: 'Gewinnrücklagen' },
-      { accountNumber: '1600', credit: 40500, description: 'Verbindlichkeiten L+L' },
-      { accountNumber: '1700', credit: 28000, description: 'Sonstige Verbindlichkeiten' },
+      { accountNumber: '9000', credit: 150000, description: 'Eigenkapital', postingKey: 40 },
+      { accountNumber: '9300', credit: 35000, description: 'Gewinnrücklagen', postingKey: 40 },
+      { accountNumber: '70001', credit: 40500, description: 'Verbindlichkeiten Lieferant', postingKey: 40 },
+      { accountNumber: '1700', credit: 28000, description: 'Sonstige Verbindlichkeiten', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -61,9 +80,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Verkauf Waren auf Rechnung',
     reference: 'RE-2024-001',
     lines: [
-      { accountNumber: '1400', debit: 11900, description: 'Forderungen inkl. USt' },
-      { accountNumber: '4300', credit: 10000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 1900, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 11900, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '4300', credit: 10000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 1900, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -74,9 +93,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Einkauf Material auf Rechnung',
     reference: 'ER-2024-001',
     lines: [
-      { accountNumber: '2100', debit: 5000, description: 'Bezogene Waren' },
-      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%' },
-      { accountNumber: '1600', credit: 5950, description: 'Verbindlichkeiten' },
+      { accountNumber: '2100', debit: 5000, description: 'Bezogene Waren', postingKey: 40 },
+      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '70001', credit: 5950, description: 'Verbindlichkeiten', postingKey: 9 },
     ],
     skrType: 'SKR04',
   });
@@ -87,9 +106,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Gehaltszahlung Januar',
     reference: 'GH-2024-01',
     lines: [
-      { accountNumber: '2300', debit: 8000, description: 'Löhne' },
-      { accountNumber: '2400', debit: 1600, description: 'Gehälter' },
-      { accountNumber: '1200', credit: 9600, description: 'Banküberweisung' },
+      { accountNumber: '2300', debit: 8000, description: 'Löhne', postingKey: 40 },
+      { accountNumber: '2400', debit: 1600, description: 'Gehälter', postingKey: 40 },
+      { accountNumber: '1200', credit: 9600, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -100,8 +119,8 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Zahlungseingang Kunde',
     reference: 'ZE-2024-001',
     lines: [
-      { accountNumber: '1200', debit: 11900, description: 'Bankgutschrift' },
-      { accountNumber: '1400', credit: 11900, description: 'Forderungsausgleich' },
+      { accountNumber: '1200', debit: 11900, description: 'Bankgutschrift', postingKey: 40 },
+      { accountNumber: '10001', credit: 11900, description: 'Forderungsausgleich', postingKey: 3 },
     ],
     skrType: 'SKR04',
   });
@@ -112,8 +131,8 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Miete Februar',
     reference: 'MI-2024-02',
     lines: [
-      { accountNumber: '3000', debit: 2000, description: 'Miete' },
-      { accountNumber: '1200', credit: 2000, description: 'Banküberweisung' },
+      { accountNumber: '3000', debit: 2000, description: 'Miete', postingKey: 40 },
+      { accountNumber: '1200', credit: 2000, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -124,9 +143,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Büromaterial',
     reference: 'BM-2024-001',
     lines: [
-      { accountNumber: '3100', debit: 200, description: 'Bürobedarf' },
-      { accountNumber: '1571', debit: 38, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 238, description: 'Bankzahlung' },
+      { accountNumber: '3100', debit: 200, description: 'Bürobedarf', postingKey: 40 },
+      { accountNumber: '1571', debit: 38, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 238, description: 'Bankzahlung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -137,9 +156,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Tankrechnung Firmenfahrzeug',
     reference: 'KFZ-2024-001',
     lines: [
-      { accountNumber: '3300', debit: 150, description: 'Kfz-Kosten' },
-      { accountNumber: '1571', debit: 28.50, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 178.50, description: 'Bankzahlung' },
+      { accountNumber: '3300', debit: 150, description: 'Kfz-Kosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 28.50, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 178.50, description: 'Bankzahlung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -150,9 +169,9 @@ tap.test('should record Q1 business transactions for SKR04', async () => {
     description: 'Verkauf Dienstleistung',
     reference: 'RE-2024-002',
     lines: [
-      { accountNumber: '1400', debit: 7140, description: 'Forderungen inkl. USt' },
-      { accountNumber: '4300', credit: 6000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 1140, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 7140, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '4300', credit: 6000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 1140, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -167,9 +186,9 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Kauf neue Produktionsmaschine',
     reference: 'INV-2024-001',
     lines: [
-      { accountNumber: '0500', debit: 25000, description: 'Neue Maschine' },
-      { accountNumber: '1571', debit: 4750, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 29750, description: 'Banküberweisung' },
+      { accountNumber: '0500', debit: 25000, description: 'Neue Maschine', postingKey: 40 },
+      { accountNumber: '1571', debit: 4750, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 29750, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -180,9 +199,9 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Großauftrag Kunde ABC',
     reference: 'RE-2024-003',
     lines: [
-      { accountNumber: '1400', debit: 35700, description: 'Forderungen inkl. USt' },
-      { accountNumber: '4300', credit: 30000, description: 'Erlöse 19% USt' },
-      { accountNumber: '1771', credit: 5700, description: 'Umsatzsteuer 19%' },
+      { accountNumber: '10001', debit: 35700, description: 'Forderungen inkl. USt', postingKey: 9 },
+      { accountNumber: '4300', credit: 30000, description: 'Erlöse 19% USt', postingKey: 40 },
+      { accountNumber: '1771', credit: 5700, description: 'Umsatzsteuer 19%', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -193,9 +212,9 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Werbekampagne',
     reference: 'WK-2024-001',
     lines: [
-      { accountNumber: '3400', debit: 5000, description: 'Werbekosten' },
-      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%' },
-      { accountNumber: '1600', credit: 5950, description: 'Verbindlichkeiten' },
+      { accountNumber: '3400', debit: 5000, description: 'Werbekosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 950, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '70001', credit: 5950, description: 'Verbindlichkeiten', postingKey: 9 },
     ],
     skrType: 'SKR04',
   });
@@ -206,9 +225,9 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Steuerberatung',
     reference: 'STB-2024-001',
     lines: [
-      { accountNumber: '3500', debit: 2500, description: 'Steuerberatungskosten' },
-      { accountNumber: '1571', debit: 475, description: 'Vorsteuer 19%' },
-      { accountNumber: '1200', credit: 2975, description: 'Banküberweisung' },
+      { accountNumber: '3500', debit: 2500, description: 'Steuerberatungskosten', postingKey: 40 },
+      { accountNumber: '1571', debit: 475, description: 'Vorsteuer 19%', postingKey: 9 },
+      { accountNumber: '1200', credit: 2975, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -219,9 +238,9 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Jahresbonus Mitarbeiter',
     reference: 'BON-2024',
     lines: [
-      { accountNumber: '2300', debit: 10000, description: 'Tantieme' },
-      { accountNumber: '2400', debit: 2000, description: 'Gehälter Bonus' },
-      { accountNumber: '1200', credit: 12000, description: 'Banküberweisung' },
+      { accountNumber: '2300', debit: 10000, description: 'Tantieme', postingKey: 40 },
+      { accountNumber: '2400', debit: 2000, description: 'Gehälter Bonus', postingKey: 40 },
+      { accountNumber: '1200', credit: 12000, description: 'Banküberweisung', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -232,8 +251,8 @@ tap.test('should record Q2-Q4 business transactions for SKR04', async () => {
     description: 'Zahlungseingang Großauftrag',
     reference: 'ZE-2024-003',
     lines: [
-      { accountNumber: '1200', debit: 35700, description: 'Bankgutschrift' },
-      { accountNumber: '1400', credit: 35700, description: 'Forderungsausgleich' },
+      { accountNumber: '1200', debit: 35700, description: 'Bankgutschrift', postingKey: 40 },
+      { accountNumber: '10001', credit: 35700, description: 'Forderungsausgleich', postingKey: 3 },
     ],
     skrType: 'SKR04',
   });
@@ -246,8 +265,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'Abschreibung Gebäude (linear 2%)',
     reference: 'AFA-2024-001',
     lines: [
-      { accountNumber: '3700', debit: 2400, description: 'AfA auf Gebäude' },
-      { accountNumber: '0210', credit: 2400, description: 'Wertberichtigung Gebäude' },
+      { accountNumber: '3700', debit: 2400, description: 'AfA auf Gebäude', postingKey: 40 },
+      { accountNumber: '0210', credit: 2400, description: 'Wertberichtigung Gebäude', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -257,8 +276,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'Abschreibung BGA (linear 10%)',
     reference: 'AFA-2024-002',
     lines: [
-      { accountNumber: '3700', debit: 6000, description: 'AfA auf BGA' }, // (35000 + 25000) * 10%
-      { accountNumber: '0500', credit: 6000, description: 'Wertberichtigung BGA' },
+      { accountNumber: '3700', debit: 6000, description: 'AfA auf BGA', postingKey: 40 }, // (35000 + 25000) * 10%
+      { accountNumber: '0500', credit: 6000, description: 'Wertberichtigung BGA', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -268,8 +287,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'Abschreibung Fuhrpark (linear 20%)',
     reference: 'AFA-2024-003',
     lines: [
-      { accountNumber: '3700', debit: 1600, description: 'AfA auf Fuhrpark' },
-      { accountNumber: '0400', credit: 1600, description: 'Wertberichtigung Fuhrpark' },
+      { accountNumber: '3700', debit: 1600, description: 'AfA auf Fuhrpark', postingKey: 40 },
+      { accountNumber: '0400', credit: 1600, description: 'Wertberichtigung Fuhrpark', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -280,8 +299,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'Aktive Rechnungsabgrenzung - Vorausbezahlte Versicherung',
     reference: 'ARA-2024-001',
     lines: [
-      { accountNumber: '1900', debit: 1000, description: 'Aktive Rechnungsabgrenzung' },
-      { accountNumber: '3200', credit: 1000, description: 'Versicherungen' },
+      { accountNumber: '1900', debit: 1000, description: 'Aktive Rechnungsabgrenzung', postingKey: 40 },
+      { accountNumber: '3200', credit: 1000, description: 'Versicherungen', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -292,8 +311,8 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'Rückstellung für Jahresabschlusskosten',
     reference: 'RS-2024-001',
     lines: [
-      { accountNumber: '3500', debit: 3000, description: 'Rechts- und Beratungskosten' },
-      { accountNumber: '0800', credit: 3000, description: 'Rückstellungen' },
+      { accountNumber: '3500', debit: 3000, description: 'Rechts- und Beratungskosten', postingKey: 40 },
+      { accountNumber: '0800', credit: 3000, description: 'Rückstellungen', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -304,9 +323,9 @@ tap.test('should perform year-end adjustments (Jahresabschlussbuchungen) for SKR
     description: 'USt-Abschluss Q4',
     reference: 'UST-2024-Q4',
     lines: [
-      { accountNumber: '1771', debit: 8740, description: 'USt-Saldo' }, // Total collected VAT
-      { accountNumber: '1571', credit: 7191.50, description: 'Vorsteuer-Saldo' }, // Total input VAT
-      { accountNumber: '1700', credit: 1548.50, description: 'USt-Zahllast' },
+      { accountNumber: '1771', debit: 8740, description: 'USt-Saldo', postingKey: 40 }, // Total collected VAT
+      { accountNumber: '1571', credit: 7191.50, description: 'Vorsteuer-Saldo', postingKey: 40 }, // Total input VAT
+      { accountNumber: '1700', credit: 1548.50, description: 'USt-Zahllast', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -360,8 +379,8 @@ tap.test('should perform closing entries (Abschlussbuchungen) for SKR04', async
     description: 'Abschluss Ertragskonten',
     reference: 'AB-2024-001',
     lines: [
-      { accountNumber: '4300', debit: 46000, description: 'Erlöse abschließen' },
-      { accountNumber: '9500', credit: 46000, description: 'GuV-Konto' },
+      { accountNumber: '4300', debit: 46000, description: 'Erlöse abschließen', postingKey: 40 },
+      { accountNumber: '9500', credit: 46000, description: 'GuV-Konto', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -372,17 +391,17 @@ tap.test('should perform closing entries (Abschlussbuchungen) for SKR04', async
     description: 'Abschluss Aufwandskonten',
     reference: 'AB-2024-002',
     lines: [
-      { accountNumber: '9500', debit: 48450, description: 'GuV-Konto' },
-      { accountNumber: '3200', debit: 1000, description: 'Versicherung abschließen (credit balance)' },
-      { accountNumber: '2100', credit: 5000, description: 'Bezogene Waren abschließen' },
-      { accountNumber: '2300', credit: 18000, description: 'Löhne abschließen' },
-      { accountNumber: '2400', credit: 3600, description: 'Gehälter abschließen' },
-      { accountNumber: '3700', credit: 10000, description: 'AfA abschließen' },
-      { accountNumber: '3000', credit: 2000, description: 'Miete abschließen' },
-      { accountNumber: '3300', credit: 150, description: 'Kfz abschließen' },
-      { accountNumber: '3400', credit: 5000, description: 'Werbung abschließen' },
-      { accountNumber: '3500', credit: 5500, description: 'Beratung abschließen' },
-      { accountNumber: '3100', credit: 200, description: 'Bürobedarf abschließen' },
+      { accountNumber: '9500', debit: 48450, description: 'GuV-Konto', postingKey: 40 },
+      { accountNumber: '3200', debit: 1000, description: 'Versicherung abschließen (credit balance)', postingKey: 40 },
+      { accountNumber: '2100', credit: 5000, description: 'Bezogene Waren abschließen', postingKey: 40 },
+      { accountNumber: '2300', credit: 18000, description: 'Löhne abschließen', postingKey: 40 },
+      { accountNumber: '2400', credit: 3600, description: 'Gehälter abschließen', postingKey: 40 },
+      { accountNumber: '3700', credit: 10000, description: 'AfA abschließen', postingKey: 40 },
+      { accountNumber: '3000', credit: 2000, description: 'Miete abschließen', postingKey: 40 },
+      { accountNumber: '3300', credit: 150, description: 'Kfz abschließen', postingKey: 40 },
+      { accountNumber: '3400', credit: 5000, description: 'Werbung abschließen', postingKey: 40 },
+      { accountNumber: '3500', credit: 5500, description: 'Beratung abschließen', postingKey: 40 },
+      { accountNumber: '3100', credit: 200, description: 'Bürobedarf abschließen', postingKey: 40 },
     ],
     skrType: 'SKR04',
   });
@@ -395,8 +414,8 @@ tap.test('should perform closing entries (Abschlussbuchungen) for SKR04', async
       description: 'Jahresgewinn auf Eigenkapital',
       reference: 'AB-2024-003',
       lines: [
-        { accountNumber: '9500', debit: guv_result, description: 'GuV-Konto ausgleichen' },
-        { accountNumber: '9300', credit: guv_result, description: 'Gewinnrücklagen' },
+        { accountNumber: '9500', debit: guv_result, description: 'GuV-Konto ausgleichen', postingKey: 40 },
+        { accountNumber: '9300', credit: guv_result, description: 'Gewinnrücklagen', postingKey: 40 },
       ],
       skrType: 'SKR04',
     });
@@ -406,8 +425,8 @@ tap.test('should perform closing entries (Abschlussbuchungen) for SKR04', async
       description: 'Jahresverlust auf Eigenkapital',
       reference: 'AB-2024-003',
       lines: [
-        { accountNumber: '9400', debit: Math.abs(guv_result), description: 'Verlustvortrag' },
-        { accountNumber: '9500', credit: Math.abs(guv_result), description: 'GuV-Konto ausgleichen' },
+        { accountNumber: '9400', debit: Math.abs(guv_result), description: 'Verlustvortrag', postingKey: 40 },
+        { accountNumber: '9500', credit: Math.abs(guv_result), description: 'GuV-Konto ausgleichen', postingKey: 40 },
       ],
       skrType: 'SKR04',
     });

test/test.skr03.ts

@@ -91,9 +91,9 @@ tap.test('should post journal entry in SKR03', async () => {
     description: 'Test journal entry',
     reference: 'JE-001',
     lines: [
-      { accountNumber: '1000', debit: 500 }, // Cash
-      { accountNumber: '1200', debit: 500 }, // Bank
-      { accountNumber: '4000', credit: 1000 }, // Revenue
+      { accountNumber: '1000', debit: 500, postingKey: 40 }, // Cash
+      { accountNumber: '1200', debit: 500, postingKey: 40 }, // Bank
+      { accountNumber: '4000', credit: 1000, postingKey: 40 }, // Revenue
     ],
     skrType: 'SKR03',
   });

test/test.skr04.ts

@@ -68,10 +68,19 @@ tap.test('should handle Class 8 as free for use in SKR04', async () => {
 });
 
 tap.test('should post complex transaction in SKR04', async () => {
+  // Create creditor account for supplier
+  await api.createAccount({
+    accountNumber: '70001',
+    accountName: 'Lieferant Test GmbH',
+    accountClass: 7,
+    accountType: 'liability',
+    skrType: 'SKR04',
+  });
+
   const transaction = await api.postTransaction({
     date: new Date(),
     debitAccount: '5400', // Goods with 19% VAT
-    creditAccount: '1600', // Trade payables
+    creditAccount: '70001', // Creditor account (supplier)
     amount: 119,
     description: 'Purchase with VAT',
     reference: 'BILL-001',

test/test.transactions.ts

@@ -29,8 +29,8 @@ tap.test('should enforce double-entry bookkeeping rules', async () => {
       description: 'Unbalanced entry',
       reference: 'TEST-001',
       lines: [
-        { accountNumber: '1000', debit: 100 },
-        { accountNumber: '4000', credit: 50 }, // Unbalanced!
+        { accountNumber: '1000', debit: 100, postingKey: 40 },
+        { accountNumber: '4000', credit: 50, postingKey: 40 }, // Unbalanced!
       ],
       skrType: 'SKR03',
     });
@@ -99,10 +99,10 @@ tap.test(
       description: 'Complex distribution',
       reference: 'COMPLEX-001',
       lines: [
-        { accountNumber: '5000', debit: 500, description: 'Materials' },
-        { accountNumber: '6000', debit: 300, description: 'Wages' },
-        { accountNumber: '7100', debit: 200, description: 'Rent' },
-        { accountNumber: '1200', credit: 1000, description: 'Bank payment' },
+        { accountNumber: '5000', debit: 500, description: 'Materials', postingKey: 40 },
+        { accountNumber: '6000', debit: 300, description: 'Wages', postingKey: 40 },
+        { accountNumber: '7100', debit: 200, description: 'Rent', postingKey: 40 },
+        { accountNumber: '1200', credit: 1000, description: 'Bank payment', postingKey: 40 },
       ],
       skrType: 'SKR03',
     });
@@ -220,10 +220,19 @@ tap.test('should handle batch transaction posting', async () => {
 });
 
 tap.test('should handle transaction with VAT', async () => {
+  // Create creditor account for supplier
+  await api.createAccount({
+    accountNumber: '70001',
+    accountName: 'Lieferant Test GmbH',
+    accountClass: 7,
+    accountType: 'liability',
+    skrType: 'SKR03',
+  });
+
   const transaction = await api.postTransaction({
     date: new Date(),
     debitAccount: '5400', // Goods with 19% VAT
-    creditAccount: '1600', // Trade payables
+    creditAccount: '70001', // Creditor account (supplier)
     amount: 119,
     description: 'Purchase including VAT',
     skrType: 'SKR03',

ts/skr.classes.account.ts

@@ -56,6 +56,9 @@ export class Account extends SmartDataDbDoc<Account, Account> {
   @svDb()
   public isSystemAccount: boolean;
 
+  @svDb()
+  public isAutomaticAccount: boolean;
+
   @svDb()
   public createdAt: Date;
 
@@ -90,6 +93,7 @@ export class Account extends SmartDataDbDoc<Account, Account> {
       this.debitTotal = 0;
       this.creditTotal = 0;
       this.isSystemAccount = true;
+      this.isAutomaticAccount = data.isAutomaticAccount || false;
       this.createdAt = new Date();
       this.updatedAt = new Date();
     }
@@ -157,6 +161,84 @@ export class Account extends SmartDataDbDoc<Account, Account> {
     );
   }
 
+  /**
+   * Check if account number is in debtor range (10000-69999)
+   * Debtor accounts (Debitorenkonten) are individual customer accounts
+   */
+  public static isInDebtorRange(accountNumber: string): boolean {
+    const num = parseInt(accountNumber);
+    return num >= 10000 && num <= 69999;
+  }
+
+  /**
+   * Check if account number is in creditor range (70000-99999)
+   * Creditor accounts (Kreditorenkonten) are individual vendor accounts
+   */
+  public static isInCreditorRange(accountNumber: string): boolean {
+    const num = parseInt(accountNumber);
+    return num >= 70000 && num <= 99999;
+  }
+
+  /**
+   * Check if account is an automatic account (Automatikkonto)
+   * Automatic accounts like 1400/1600 cannot be posted to directly
+   */
+  public static isAutomaticAccount(accountNumber: string, skrType: TSKRType): boolean {
+    // SKR03: 1400 (Forderungen), 1600 (Verbindlichkeiten)
+    // SKR04: 1400 (Forderungen), 1600 (Verbindlichkeiten), 3300 (Alternative Verbindlichkeiten)
+    if (skrType === 'SKR03') {
+      return accountNumber === '1400' || accountNumber === '1600';
+    } else {
+      return accountNumber === '1400' || accountNumber === '1600' || accountNumber === '3300';
+    }
+  }
+
+  /**
+   * Validate account for posting - throws error if account cannot be posted to
+   */
+  public static async validateAccountForPosting(
+    accountNumber: string,
+    skrType: TSKRType,
+  ): Promise<void> {
+    // Check if automatic account
+    if (Account.isAutomaticAccount(accountNumber, skrType)) {
+      throw new Error(
+        `Account ${accountNumber} is an automatic account (Automatikkonto) and cannot be posted to directly. ` +
+        `Use debtor accounts (10000-69999) or creditor accounts (70000-99999) instead.`
+      );
+    }
+
+    // Get account to verify it exists
+    const account = await Account.getAccountByNumber(accountNumber, skrType);
+    if (!account) {
+      throw new Error(
+        `Account ${accountNumber} not found in ${skrType}. ` +
+        `Please create the account before posting.`
+      );
+    }
+
+    // Check if account is active
+    if (!account.isActive) {
+      throw new Error(
+        `Account ${accountNumber} is inactive and cannot be posted to.`
+      );
+    }
+  }
+
+  /**
+   * Check if this account instance is a debtor account
+   */
+  public isDebtorAccount(): boolean {
+    return Account.isInDebtorRange(this.accountNumber);
+  }
+
+  /**
+   * Check if this account instance is a creditor account
+   */
+  public isCreditorAccount(): boolean {
+    return Account.isInCreditorRange(this.accountNumber);
+  }
+
   public async updateBalance(
     debitAmount: number = 0,
     creditAmount: number = 0,
@@ -209,19 +291,33 @@ export class Account extends SmartDataDbDoc<Account, Account> {
 
   public async beforeSave(): Promise<void> {
     // Validate account number format
-    if (!this.accountNumber || this.accountNumber.length !== 4) {
+    const accountLength = this.accountNumber?.length || 0;
+    if (!this.accountNumber || (accountLength !== 4 && accountLength !== 5)) {
       throw new Error(
-        `Invalid account number format: ${this.accountNumber}. Must be 4 digits.`,
+        `Invalid account number format: ${this.accountNumber}. Must be 4 digits (standard SKR) or 5 digits (debtor/creditor).`,
       );
     }
 
     // Validate account number is numeric
-    if (!/^\d{4}$/.test(this.accountNumber)) {
+    if (!/^\d{4,5}$/.test(this.accountNumber)) {
       throw new Error(
         `Account number must contain only digits: ${this.accountNumber}`,
       );
     }
 
+    // For 5-digit accounts, validate they are in debtor (10000-69999) or creditor (70000-99999) ranges
+    if (accountLength === 5) {
+      const accountNum = parseInt(this.accountNumber);
+      const isDebtor = accountNum >= 10000 && accountNum <= 69999;
+      const isCreditor = accountNum >= 70000 && accountNum <= 99999;
+      
+      if (!isDebtor && !isCreditor) {
+        throw new Error(
+          `5-digit account number ${this.accountNumber} must be in debtor range (10000-69999) or creditor range (70000-99999).`,
+        );
+      }
+    }
+
     // Validate account class matches first digit
     const firstDigit = parseInt(this.accountNumber[0]);
     if (this.accountClass !== firstDigit) {
@@ -234,5 +330,11 @@ export class Account extends SmartDataDbDoc<Account, Account> {
     if (this.skrType !== 'SKR03' && this.skrType !== 'SKR04') {
       throw new Error(`Invalid SKR type: ${this.skrType}`);
     }
+
+    // Mark automatic accounts (Automatikkonten)
+    // These are summary accounts that cannot be posted to directly
+    if (Account.isAutomaticAccount(this.accountNumber, this.skrType)) {
+      this.isAutomaticAccount = true;
+    }
   }
 }

ts/skr.classes.journalentry.ts

@@ -2,6 +2,11 @@ import * as plugins from './plugins.js';
 import { getDbSync } from './skr.database.js';
 import { Account } from './skr.classes.account.js';
 import { Transaction } from './skr.classes.transaction.js';
+import {
+  validatePostingKey,
+  validatePostingKeyConsistency,
+  getPostingKeyDescription,
+} from './skr.postingkeys.js';
 import type {
   TSKRType,
   IJournalEntry,
@@ -212,22 +217,84 @@ export class JournalEntry extends SmartDataDbDoc<JournalEntry, JournalEntry> {
       throw new Error('Journal entry must have at least 2 lines');
     }
 
-    // Validate all accounts exist and are active
+    // Validate all accounts exist, are active, and can be posted to
+    const validationErrors: string[] = [];
+    const validationWarnings: string[] = [];
+
     for (const line of this.lines) {
+      // Validate posting key is present (REQUIRED)
+      if (!line.postingKey) {
+        validationErrors.push(
+          `Line for account ${line.accountNumber} is missing required posting key (Buchungsschlüssel). ` +
+          `Posting keys are mandatory for DATEV compliance.`
+        );
+        continue; // Skip further validation for this line
+      }
+
+      // Validate account is not an automatic account (Automatikkonto)
+      try {
+        await Account.validateAccountForPosting(line.accountNumber, this.skrType);
+      } catch (error) {
+        validationErrors.push(error.message);
+        continue; // Skip further validation for this line
+      }
+
+      // Get account for posting key validation
       const account = await Account.getAccountByNumber(
         line.accountNumber,
         this.skrType,
       );
 
       if (!account) {
-        throw new Error(
+        validationErrors.push(
           `Account ${line.accountNumber} not found for ${this.skrType}`,
         );
+        continue;
       }
 
       if (!account.isActive) {
-        throw new Error(`Account ${line.accountNumber} is not active`);
+        validationErrors.push(`Account ${line.accountNumber} is not active`);
+        continue;
       }
+
+      // Validate posting key for this line
+      const amount = line.debit || line.credit || 0;
+      const postingKeyValidation = validatePostingKey(
+        line.postingKey,
+        line.accountNumber,
+        amount
+      );
+
+      if (!postingKeyValidation.isValid) {
+        validationErrors.push(...postingKeyValidation.errors);
+      }
+
+      if (postingKeyValidation.warnings.length > 0) {
+        validationWarnings.push(...postingKeyValidation.warnings);
+      }
+    }
+
+    // Validate posting key consistency across all lines
+    const consistencyValidation = validatePostingKeyConsistency(this.lines);
+    if (!consistencyValidation.isValid) {
+      validationErrors.push(...consistencyValidation.errors);
+    }
+    if (consistencyValidation.warnings.length > 0) {
+      validationWarnings.push(...consistencyValidation.warnings);
+    }
+
+    // Log warnings but don't fail validation
+    if (validationWarnings.length > 0) {
+      console.warn('Journal entry validation warnings:');
+      validationWarnings.forEach(warning => console.warn(`  - ${warning}`));
+    }
+
+    // Throw if any errors
+    if (validationErrors.length > 0) {
+      throw new Error(
+        'Journal entry validation failed:\n' +
+        validationErrors.map(e => `  - ${e}`).join('\n')
+      );
     }
   }
 
@@ -325,6 +392,7 @@ export class JournalEntry extends SmartDataDbDoc<JournalEntry, JournalEntry> {
       credit: line.debit, // Swap
       description: `Reversal: ${line.description || ''}`,
       costCenter: line.costCenter,
+      postingKey: line.postingKey, // Keep same posting key for reversal
     }));
 
     const reversalEntry = new JournalEntry({

ts/skr.classes.ledger.ts

@@ -418,6 +418,7 @@ export class Ledger {
           accountNumber: account.accountNumber,
           debit: Math.abs(balance),
           description: `Closing ${account.accountName}`,
+          postingKey: 40, // Tax-free - internal closing entry
         });
         totalRevenue += Math.abs(balance);
       }
@@ -429,6 +430,7 @@ export class Ledger {
         accountNumber: closingAccountNumber,
         credit: totalRevenue,
         description: 'Revenue closing to P&L',
+        postingKey: 40, // Tax-free - internal closing entry
       });
 
       const revenueClosingEntry = await this.postJournalEntry({
@@ -458,6 +460,7 @@ export class Ledger {
           accountNumber: account.accountNumber,
           credit: Math.abs(balance),
           description: `Closing ${account.accountName}`,
+          postingKey: 40, // Tax-free - internal closing entry
         });
         totalExpense += Math.abs(balance);
       }
@@ -469,6 +472,7 @@ export class Ledger {
         accountNumber: closingAccountNumber,
         debit: totalExpense,
         description: 'Expense closing to P&L',
+        postingKey: 40, // Tax-free - internal closing entry
       });
 
       const expenseClosingEntry = await this.postJournalEntry({

ts/skr.invoice.booking.ts

@@ -1,6 +1,7 @@
 import * as plugins from './plugins.js';
 import { JournalEntry } from './skr.classes.journalentry.js';
 import { SKRInvoiceMapper } from './skr.invoice.mapper.js';
+import { suggestPostingKey } from './skr.postingkeys.js';
 import type { TSKRType, IJournalEntry, IJournalEntryLine } from './skr.types.js';
 import type {
   IInvoice,
@@ -196,14 +197,16 @@ export class InvoiceBookingEngine {
         lines.push({
           accountNumber,
           credit: Math.abs(amount),
-          description: this.getAccountDescription(accountNumber, group)
+          description: this.getAccountDescription(accountNumber, group),
+          postingKey: 9 // 19% input VAT for expenses
         });
       } else {
         // Regular invoice: debit expense account
         lines.push({
           accountNumber,
           debit: Math.abs(amount),
-          description: this.getAccountDescription(accountNumber, group)
+          description: this.getAccountDescription(accountNumber, group),
+          postingKey: 9 // 19% input VAT for expenses
         });
       }
     }
@@ -221,14 +224,16 @@ export class InvoiceBookingEngine {
       lines.push({
         accountNumber: controlAccount,
         debit: totalAmount,
-        description: `${invoice.supplier.name} - Credit Note ${invoice.invoiceNumber}`
+        description: `${invoice.supplier.name} - Credit Note ${invoice.invoiceNumber}`,
+        postingKey: 40 // Tax-free for control account
       });
     } else {
       // Regular invoice: credit vendor account
       lines.push({
         accountNumber: controlAccount,
         credit: totalAmount,
-        description: `${invoice.supplier.name} - Invoice ${invoice.invoiceNumber}`
+        description: `${invoice.supplier.name} - Invoice ${invoice.invoiceNumber}`,
+        postingKey: 40 // Tax-free for control account
       });
     }
     
@@ -257,14 +262,16 @@ export class InvoiceBookingEngine {
         lines.push({
           accountNumber,
           debit: Math.abs(amount),
-          description: this.getAccountDescription(accountNumber, group)
+          description: this.getAccountDescription(accountNumber, group),
+          postingKey: 9 // 19% output VAT for revenue
         });
       } else {
         // Regular invoice: credit revenue account
         lines.push({
           accountNumber,
           credit: Math.abs(amount),
-          description: this.getAccountDescription(accountNumber, group)
+          description: this.getAccountDescription(accountNumber, group),
+          postingKey: 9 // 19% output VAT for revenue
         });
       }
     }
@@ -282,14 +289,16 @@ export class InvoiceBookingEngine {
       lines.push({
         accountNumber: controlAccount,
         credit: totalAmount,
-        description: `${invoice.customer.name} - Credit Note ${invoice.invoiceNumber}`
+        description: `${invoice.customer.name} - Credit Note ${invoice.invoiceNumber}`,
+        postingKey: 40 // Tax-free for control account
       });
     } else {
       // Regular invoice: debit customer account
       lines.push({
         accountNumber: controlAccount,
         debit: totalAmount,
-        description: `${invoice.customer.name} - Invoice ${invoice.invoiceNumber}`
+        description: `${invoice.customer.name} - Invoice ${invoice.invoiceNumber}`,
+        postingKey: 40 // Tax-free for control account
       });
     }
     
@@ -325,20 +334,23 @@ export class InvoiceBookingEngine {
       
       const amount = Math.abs(vatBreak.taxAmount);
       const description = `VAT ${vatBreak.vatCategory.rate}%`;
+      const vatRate = vatBreak.vatCategory.rate;
+      // Select posting key based on VAT rate: 8 for 7%, 9 for 19%
+      const postingKey = vatRate === 7 ? 8 : 9;
 
       if (direction === 'input') {
         // Input VAT (Vorsteuer)
         if (reverseDirection) {
-          lines.push({ accountNumber: vatAccount, credit: amount, description });
+          lines.push({ accountNumber: vatAccount, credit: amount, description, postingKey });
         } else {
-          lines.push({ accountNumber: vatAccount, debit: amount, description });
+          lines.push({ accountNumber: vatAccount, debit: amount, description, postingKey });
         }
       } else {
         // Output VAT (Umsatzsteuer)
         if (reverseDirection) {
-          lines.push({ accountNumber: vatAccount, debit: amount, description });
+          lines.push({ accountNumber: vatAccount, debit: amount, description, postingKey });
         } else {
-          lines.push({ accountNumber: vatAccount, credit: amount, description });
+          lines.push({ accountNumber: vatAccount, credit: amount, description, postingKey });
         }
       }
     }
@@ -404,12 +416,14 @@ export class InvoiceBookingEngine {
         {
           accountNumber: inputVATAccount,
           debit: amount,
-          description: `Reverse charge input VAT ${vatBreak.vatCategory.rate}%`
+          description: `Reverse charge input VAT ${vatBreak.vatCategory.rate}%`,
+          postingKey: 94 // Reverse charge posting key
         },
         {
           accountNumber: outputVATAccount,
           credit: amount,
-          description: `Reverse charge output VAT ${vatBreak.vatCategory.rate}%`
+          description: `Reverse charge output VAT ${vatBreak.vatCategory.rate}%`,
+          postingKey: 94 // Reverse charge posting key
         }
       );
     }
@@ -462,12 +476,14 @@ export class InvoiceBookingEngine {
           {
             accountNumber: controlAccount,
             debit: fullAmount,
-            description: `Payment to ${invoice.supplier.name}`
+            description: `Payment to ${invoice.supplier.name}`,
+            postingKey: 3 // Payment with VAT
           },
           {
             accountNumber: '1000', // Bank account (would be configurable)
             credit: paymentAmount,
-            description: `Bank payment ${payment.endToEndId || payment.paymentId}`
+            description: `Bank payment ${payment.endToEndId || payment.paymentId}`,
+            postingKey: 40 // Tax-free for bank account
           }
         );
 
@@ -477,7 +493,8 @@ export class InvoiceBookingEngine {
           lines.push({
             accountNumber: skontoAccounts.skontoAccount,
             credit: skontoAmount,
-            description: `Skonto received`
+            description: `Skonto received`,
+            postingKey: 40 // Tax-free for skonto
           });
 
           // VAT correction for skonto
@@ -488,7 +505,8 @@ export class InvoiceBookingEngine {
               {
                 accountNumber: skontoAccounts.vatCorrectionAccount,
                 credit: vatCorrection,
-                description: `Skonto VAT correction`
+                description: `Skonto VAT correction`,
+                postingKey: 40 // Tax-free for correction
               }
             );
           }
@@ -499,12 +517,14 @@ export class InvoiceBookingEngine {
           {
             accountNumber: '1000', // Bank account
             debit: paymentAmount,
-            description: `Payment from ${invoice.customer.name}`
+            description: `Payment from ${invoice.customer.name}`,
+            postingKey: 40 // Tax-free for bank account
           },
           {
             accountNumber: controlAccount,
             credit: fullAmount,
-            description: `Customer payment ${payment.endToEndId || payment.paymentId}`
+            description: `Customer payment ${payment.endToEndId || payment.paymentId}`,
+            postingKey: 3 // Payment with VAT
           }
         );
 
@@ -514,7 +534,8 @@ export class InvoiceBookingEngine {
           lines.push({
             accountNumber: skontoAccounts.skontoAccount,
             debit: skontoAmount,
-            description: `Skonto granted`
+            description: `Skonto granted`,
+            postingKey: 40 // Tax-free for skonto
           });
 
           // VAT correction for skonto
@@ -525,7 +546,8 @@ export class InvoiceBookingEngine {
               {
                 accountNumber: skontoAccounts.vatCorrectionAccount,
                 debit: vatCorrection,
-                description: `Skonto VAT correction`
+                description: `Skonto VAT correction`,
+                postingKey: 40 // Tax-free for correction
               }
             );
           }

ts/skr.postingkeys.ts

@@ -0,0 +1,245 @@
+/**
+ * DATEV Posting Keys (Buchungsschlüssel) for German Accounting
+ *
+ * Posting keys control automatic VAT booking and are automatically checked
+ * in German tax audits (Betriebsprüfungen). Using incorrect posting keys
+ * can have serious tax consequences.
+ *
+ * Reference: DATEV Buchungsschlüssel-Verzeichnis
+ */
+
+import type { TPostingKey, IPostingKeyRule } from './skr.types.js';
+
+/**
+ * Posting key definitions with validation rules
+ */
+export const POSTING_KEY_RULES: Record<TPostingKey, IPostingKeyRule> = {
+  3: {
+    key: 3,
+    description: 'Zahlungseingang mit 19% Umsatzsteuer',
+    vatRate: 19,
+    requiresVAT: true,
+    disablesVATAutomatism: false,
+    allowedScenarios: ['domestic_taxed']
+  },
+  8: {
+    key: 8,
+    description: '7% Vorsteuer',
+    vatRate: 7,
+    requiresVAT: true,
+    disablesVATAutomatism: false,
+    allowedScenarios: ['domestic_taxed']
+  },
+  9: {
+    key: 9,
+    description: '19% Vorsteuer',
+    vatRate: 19,
+    requiresVAT: true,
+    disablesVATAutomatism: false,
+    allowedScenarios: ['domestic_taxed']
+  },
+  19: {
+    key: 19,
+    description: '19% Vorsteuer bei innergemeinschaftlichen Lieferungen',
+    vatRate: 19,
+    requiresVAT: true,
+    disablesVATAutomatism: false,
+    allowedScenarios: ['intra_eu']
+  },
+  40: {
+    key: 40,
+    description: 'Steuerfrei / Aufhebung der Automatik',
+    vatRate: 0,
+    requiresVAT: false,
+    disablesVATAutomatism: true,
+    allowedScenarios: ['tax_free', 'export', 'reverse_charge']
+  },
+  94: {
+    key: 94,
+    description: '19% Vorsteuer/Umsatzsteuer bei Erwerb aus EU oder Drittland (Reverse Charge)',
+    vatRate: 19,
+    requiresVAT: true,
+    disablesVATAutomatism: false,
+    allowedScenarios: ['reverse_charge', 'intra_eu', 'third_country']
+  }
+};
+
+/**
+ * Validate posting key for a journal entry line
+ */
+export function validatePostingKey(
+  postingKey: TPostingKey,
+  accountNumber: string,
+  amount: number,
+  vatAmount?: number,
+  taxScenario?: string
+): { isValid: boolean; errors: string[]; warnings: string[] } {
+  const errors: string[] = [];
+  const warnings: string[] = [];
+
+  // Get posting key rule
+  const rule = POSTING_KEY_RULES[postingKey];
+  if (!rule) {
+    errors.push(`Invalid posting key: ${postingKey}`);
+    return { isValid: false, errors, warnings };
+  }
+
+  // Validate VAT requirement
+  if (rule.requiresVAT && !vatAmount) {
+    errors.push(
+      `Posting key ${postingKey} requires VAT amount, but none provided. ` +
+      `Description: ${rule.description}`
+    );
+  }
+
+  // Validate VAT rate if specified
+  if (rule.vatRate && vatAmount && rule.vatRate > 0) {
+    const expectedVAT = Math.round(amount * rule.vatRate) / 100;
+    const tolerance = 0.02; // 2 cent tolerance for rounding
+
+    if (Math.abs(vatAmount - expectedVAT) > tolerance) {
+      warnings.push(
+        `VAT amount ${vatAmount} does not match expected ${expectedVAT.toFixed(2)} ` +
+        `for posting key ${postingKey} (${rule.vatRate}%)`
+      );
+    }
+  }
+
+  // Validate tax scenario
+  if (rule.allowedScenarios && taxScenario) {
+    if (!rule.allowedScenarios.includes(taxScenario)) {
+      errors.push(
+        `Posting key ${postingKey} is not valid for tax scenario '${taxScenario}'. ` +
+        `Allowed scenarios: ${rule.allowedScenarios.join(', ')}`
+      );
+    }
+  }
+
+  // Validate automatism disabling
+  if (rule.disablesVATAutomatism && vatAmount && vatAmount > 0) {
+    warnings.push(
+      `Posting key ${postingKey} disables VAT automatism but VAT amount is provided. ` +
+      `This may cause incorrect tax reporting.`
+    );
+  }
+
+  return {
+    isValid: errors.length === 0,
+    errors,
+    warnings
+  };
+}
+
+/**
+ * Get posting key description
+ */
+export function getPostingKeyDescription(postingKey: TPostingKey): string {
+  const rule = POSTING_KEY_RULES[postingKey];
+  return rule ? rule.description : `Unknown posting key: ${postingKey}`;
+}
+
+/**
+ * Get appropriate posting key for a transaction
+ */
+export function suggestPostingKey(params: {
+  vatRate: number;
+  taxScenario?: string;
+  isPayment?: boolean;
+}): TPostingKey {
+  const { vatRate, taxScenario, isPayment } = params;
+
+  // Tax-free or reverse charge scenarios
+  if (taxScenario === 'tax_free' || taxScenario === 'export') {
+    return 40;
+  }
+
+  // Reverse charge
+  if (taxScenario === 'reverse_charge' || taxScenario === 'third_country') {
+    return 94;
+  }
+
+  // Intra-EU with VAT
+  if (taxScenario === 'intra_eu' && vatRate === 19) {
+    return 19;
+  }
+
+  // Payment with 19% VAT
+  if (isPayment && vatRate === 19) {
+    return 3;
+  }
+
+  // Input VAT based on rate
+  if (vatRate === 19) {
+    return 9;
+  }
+
+  if (vatRate === 7) {
+    return 8;
+  }
+
+  // Default to tax-free if no VAT
+  if (vatRate === 0) {
+    return 40;
+  }
+
+  // Fallback to 19% input VAT
+  return 9;
+}
+
+/**
+ * Validate all posting keys for consistency
+ */
+export function validatePostingKeyConsistency(lines: Array<{
+  postingKey: TPostingKey;
+  accountNumber: string;
+  debit?: number;
+  credit?: number;
+  vatAmount?: number;
+}>): { isValid: boolean; errors: string[]; warnings: string[] } {
+  const errors: string[] = [];
+  const warnings: string[] = [];
+
+  // Check for mixing tax-free and taxed transactions
+  const hasTaxFree = lines.some(line => line.postingKey === 40);
+  const hasTaxed = lines.some(line => [3, 8, 9, 19, 94].includes(line.postingKey));
+
+  if (hasTaxFree && hasTaxed) {
+    warnings.push(
+      'Journal entry mixes tax-free (key 40) and taxed transactions. ' +
+      'Verify this is intentional.'
+    );
+  }
+
+  // Check for reverse charge consistency
+  const hasReverseCharge = lines.some(line => line.postingKey === 94);
+  if (hasReverseCharge) {
+    const reverseChargeLines = lines.filter(line => line.postingKey === 94);
+    if (reverseChargeLines.length % 2 !== 0) {
+      errors.push(
+        'Reverse charge (posting key 94) requires both input and output VAT entries. ' +
+        'Found odd number of reverse charge lines.'
+      );
+    }
+  }
+
+  return {
+    isValid: errors.length === 0,
+    errors,
+    warnings
+  };
+}
+
+/**
+ * Check if posting key requires automatic VAT booking
+ */
+export function requiresAutomaticVAT(postingKey: TPostingKey): boolean {
+  const rule = POSTING_KEY_RULES[postingKey];
+  return rule ? !rule.disablesVATAutomatism : false;
+}
+
+/**
+ * Get all valid posting keys
+ */
+export function getAllPostingKeys(): TPostingKey[] {
+  return Object.keys(POSTING_KEY_RULES).map(k => Number(k) as TPostingKey);
+}

ts/skr.types.ts

@@ -9,6 +9,18 @@ export type TSKRType = 'SKR03' | 'SKR04';
 
 export type TTransactionStatus = 'pending' | 'posted' | 'reversed';
 
+/**
+ * DATEV posting keys (Buchungsschlüssel) for German accounting
+ * These keys control automatic VAT booking and are checked in tax audits
+ */
+export type TPostingKey =
+  | 3   // Payment with 19% VAT
+  | 8   // 7% input VAT
+  | 9   // 19% input VAT
+  | 19  // 19% input VAT (intra-EU)
+  | 40  // Tax-free (disables VAT automatism)
+  | 94; // 19% input/output VAT (reverse charge)
+
 export type TReportType =
   | 'trial_balance'
   | 'income_statement'
@@ -16,6 +28,18 @@ export type TReportType =
   | 'general_ledger'
   | 'cash_flow';
 
+/**
+ * Posting key validation rule
+ */
+export interface IPostingKeyRule {
+  key: TPostingKey;
+  description: string;
+  vatRate?: number;              // Expected VAT rate (if applicable)
+  requiresVAT: boolean;           // Whether VAT entry is required
+  disablesVATAutomatism: boolean; // Whether this key disables automatic VAT
+  allowedScenarios?: string[];    // Allowed tax scenarios (e.g., 'reverse_charge')
+}
+
 export interface IAccountData {
   accountNumber: string;
   accountName: string;
@@ -25,6 +49,7 @@ export interface IAccountData {
   description?: string;
   vatRate?: number;
   isActive?: boolean;
+  isAutomaticAccount?: boolean; // Automatikkonto (e.g., 1400, 1600) - cannot be posted to directly
 }
 
 export interface ITransactionData {
@@ -53,6 +78,7 @@ export interface IJournalEntryLine {
   credit?: number;
   description?: string;
   costCenter?: string;
+  postingKey: TPostingKey; // REQUIRED: DATEV posting key for VAT automation control
 }
 
 export interface ITrialBalanceEntry {

ts/skr03.data.ts

@@ -159,6 +159,7 @@ export const SKR03_ACCOUNTS: IAccountData[] = [
     accountType: 'asset',
     skrType: 'SKR03',
     description: 'Trade receivables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use debtor accounts (10000-69999)
   },
   {
     accountNumber: '1500',
@@ -199,6 +200,7 @@ export const SKR03_ACCOUNTS: IAccountData[] = [
     accountType: 'liability',
     skrType: 'SKR03',
     description: 'Trade payables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use creditor accounts (70000-99999)
   },
   {
     accountNumber: '1700',

ts/skr04.data.ts

@@ -159,6 +159,7 @@ export const SKR04_ACCOUNTS: IAccountData[] = [
     accountType: 'asset',
     skrType: 'SKR04',
     description: 'Trade receivables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use debtor accounts (10000-69999)
   },
   {
     accountNumber: '1500',
@@ -199,6 +200,7 @@ export const SKR04_ACCOUNTS: IAccountData[] = [
     accountType: 'liability',
     skrType: 'SKR04',
     description: 'Trade payables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use creditor accounts (70000-99999)
   },
   {
     accountNumber: '1700',