Hello Odoo Team and Community,

I'm encountering a critical issue in Odoo 18 when processing PEPPOL self-billing invoices (InvoiceTypeCode 389) that include a cac:AllowanceCharge with a cbc:BaseAmount.

When a self-billing invoice contains:

<cac:AllowanceCharge>
    <cbc:ChargeIndicator>false</cbc:ChargeIndicator>
    <cbc:Amount currencyID="EUR">10.00</cbc:Amount>
    <cbc:BaseAmount currencyID="EUR">100.00</cbc:BaseAmount>
</cac:AllowanceCharge>

Odoo misinterprets the allowance, resulting in:

• The allowance being applied as a full deduction on the line amount, instead of a percentage-based reduction.
• The taxable base not being adjusted correctly.
• The VAT calculation becoming inconsistent (e.g., VAT is calculated on incorrect net amounts).
• In some cases, the allowance appears as a negative line of -100.00 EUR, even though the intended discount is only 10% of 100.00 EUR.

This leads to incorrect total amounts and VAT reporting, which breaks compliance.