[5eBeKs]

What is wrong

build_peppol_ubl_invoice_xml writes the invoice total into PayableAmount (BT-115) and never emits PrepaidAmount (BT-113). When payments are already recorded on the invoice (a deposit, a partial payment, or a resend after a payment), the UBL that is sent through the access point or downloaded from the UBL export (/invoices/<id>/export/ubl) asks the buyer to pay the full total.

The Factur-X/ZUGFeRD export (app/utils/cii_invoice.py) already handles this: it writes amount_paid as TotalPrepaidAmount and DuePayableAmount = total - amount_paid (see test_cii_prepaid_amount in tests/test_cii_invoice.py). So the same invoice gets two different payable amounts depending on the format:

invoice	amount_paid	Factur-X DuePayableAmount	UBL PayableAmount (develop)	UBL PayableAmount (this change)
242.00 EUR	0.00	242.00	242.00	242.00
242.00 EUR	50.00	192.00	242.00	192.00 (PrepaidAmount 50.00)
242.00 EUR	242.00	0.00	242.00	0.00 (PrepaidAmount 242.00)

EN 16931 rule BR-CO-16 defines the payable amount as TaxInclusiveAmount - PrepaidAmount + PayableRoundingAmount.

What this changes

• app/integrations/peppol.py: when amount_paid is above zero, PrepaidAmount is added to LegalMonetaryTotal (before PayableAmount, as the UBL 2.1 sequence requires), and PayableAmount is the total minus the prepaid amount.
• Overpaid invoices: as in the Factur-X export, the amount due is never negative (PayableAmount 0.00). The Factur-X export still writes the full payment as TotalPrepaidAmount; here the prepaid amount is capped at the total instead, so that BR-CO-16 holds in the UBL (300.00 paid on 242.00 gives PrepaidAmount 242.00 and PayableAmount 0.00). The other UBL option, the full payment as PrepaidAmount with a negative PayableAmount, also satisfies BR-CO-16; if you would rather have that, it is a one-line change and I can switch it.
• Unpaid invoices produce the same XML as before (no PrepaidAmount, PayableAmount = total).

Unchanged: the Factur-X/CII export, the invoice totals, and the send/export routes.

Tests

New file tests/test_peppol_ubl_totals.py:

test	develop	this change
test_unpaid_invoice_payable_is_the_total_without_prepaid	passes	passes
test_missing_amount_paid_is_treated_as_unpaid	passes	passes
test_partially_paid_invoice_reports_prepaid_and_the_balance (50.00 of 242.00 → payable 192.00)	fails (no PrepaidAmount)	passes
test_prepaid_amount_precedes_payable_amount	fails (no PrepaidAmount)	passes
test_fully_paid_invoice_has_nothing_payable	fails (no PrepaidAmount)	passes
test_overpaid_invoice_has_nothing_payable_and_stays_balanced (300.00 of 242.00)	fails (no PrepaidAmount)	passes

• Peppol, e-invoice and invoice test files (test_peppol_attachments.py, test_peppol_bridge.py, test_peppol_identifiers.py, test_peppol_service.py, test_peppol_ubl_totals.py, test_invoice_validators.py, test_invoice_pdf_postprocess.py, test_zugferd.py, test_cii_invoice.py, test_invoices.py): 173 passed, 1 skipped with the change; on develop the same files give 169 passed, 1 skipped and only the 4 new tests failing.
• Full suite (Python 3.11, SQLite, no network), run on an earlier revision of this commit that differed only in the wording of a code comment, the CHANGELOG line and the commit message: 2682 passed, 5 failed, 83 skipped, 1 xfailed with the change; develop gives 2676 passed and the same 5 failures (test_time_format_display.py::test_parse_user_time_input_compact_hhmm and four tests in tests/test_utils/test_webhook_service.py), so nothing new fails.
• black, isort and flake8 are clean on the changed Python files.