Support parsing advance credit notes

Author: pmenendz

invoice.go

@@ -550,6 +550,11 @@ func (inv *Inv) parsePrepaymentTotals(goblInv *bill.Invoice) error {
 		{inv.DomesticReverseChargeNetSale, "", "10", tax.KeyReverseCharge, nil},
 	}
 
+	// For credit notes (e.g. KOR_ZAL), KSeF P_13/P_14/P_15 values are negative.
+	// GOBL expects positive totals, so we invert them. Invoice.Invert() cannot
+	// be used with the bypass tag, so we invert each amount individually.
+	isCreditNote := goblInv.Type == bill.InvoiceTypeCreditNote
+
 	var rates []*tax.RateTotal
 	var netSum, taxSum num.Amount
 
@@ -562,6 +567,9 @@ func (inv *Inv) parsePrepaymentTotals(goblInv *bill.Invoice) error {
 		if err != nil {
 			return fmt.Errorf("parsing prepayment net for category %s: %w", e.category, err)
 		}
+		if isCreditNote {
+			netAmt = netAmt.Invert()
+		}
 
 		rt := &tax.RateTotal{
 			Key:     e.key,
@@ -577,6 +585,9 @@ func (inv *Inv) parsePrepaymentTotals(goblInv *bill.Invoice) error {
 			if err != nil {
 				return fmt.Errorf("parsing prepayment tax for category %s: %w", e.category, err)
 			}
+			if isCreditNote {
+				taxAmt = taxAmt.Invert()
+			}
 			rt.Amount = taxAmt
 			taxSum = taxSum.MatchPrecision(taxAmt)
 			taxSum = taxSum.Add(taxAmt)
@@ -613,6 +624,9 @@ func (inv *Inv) parsePrepaymentTotals(goblInv *bill.Invoice) error {
 		if err != nil {
 			return fmt.Errorf("parsing total amount due: %w", err)
 		}
+		if isCreditNote {
+			payable = payable.Invert()
+		}
 		totals.Payable = payable
 	} else {
 		totals.Payable = totals.TotalWithTax

invoice_test.go

@@ -1047,6 +1047,36 @@ func TestPrepaymentEndToEnd(t *testing.T) {
 		assert.Equal(t, cbc.Code("ZAL-001"), inv.Preceding[0].Code)
 	})
 
+	t.Run("KOR_ZAL inverts negative totals for credit note", func(t *testing.T) {
+		doc := testPrepaymentDoc()
+		doc.Inv.InvoiceType = "KOR_ZAL"
+		doc.Inv.CorrectedInv = []*ksef.CorrectedInv{
+			{SequentialNumber: "ZAL-001", IssueDate: "2026-01-15"},
+		}
+		// KSeF credit notes have negative P_13/P_14/P_15 values
+		doc.Inv.StandardRateNetSale = "-1000.00"
+		doc.Inv.StandardRateTax = "-230.00"
+		doc.Inv.TotalAmountDue = "-1230.00"
+
+		inv, err := doc.ToGOBL()
+		require.NoError(t, err)
+
+		assert.True(t, inv.HasTags(tax.TagBypass))
+		assert.Equal(t, bill.InvoiceTypeCreditNote, inv.Type)
+
+		// Totals should be positive (GOBL convention)
+		assert.Equal(t, "1000.00", inv.Totals.Sum.String())
+		assert.Equal(t, "230.00", inv.Totals.Tax.String())
+		assert.Equal(t, "1230.00", inv.Totals.TotalWithTax.String())
+		assert.Equal(t, "1230.00", inv.Totals.Payable.String())
+
+		// Tax rate amounts should also be positive
+		require.NotNil(t, inv.Totals.Taxes)
+		rate := inv.Totals.Taxes.Categories[0].Rates[0]
+		assert.Equal(t, "1000.00", rate.Base.String())
+		assert.Equal(t, "230.00", rate.Amount.String())
+	})
+
 	t.Run("zero rate prepayment sets correct totals", func(t *testing.T) {
 		doc := testPrepaymentDoc()
 		doc.Inv.StandardRateNetSale = ""