Allow setting due amount according to current payments (#8)

* Allow setting due amount according to current payments

* No negative amounts

pretix_zugferd/invoice.py

@@ -19,6 +19,7 @@
 from drafthorse.models.tradelines import LineItem
 from drafthorse.pdf import attach_xml
 from pretix.base.invoice import ClassicInvoiceRenderer, Modern1Renderer
+from pretix.base.models import Order
 from pretix.base.models.tax import EU_COUNTRIES
 from reportlab.lib.units import mm
 from reportlab.pdfgen.canvas import Canvas
@@ -380,6 +381,19 @@ def _zugferd_generate_document(self, invoice):
         doc.trade.settlement.monetary_summation.tax_total = (taxtotal, cc)
         doc.trade.settlement.monetary_summation.grand_total = total
         doc.trade.settlement.monetary_summation.due_amount = total
+
+        if not self.invoice.is_cancellation:
+            payment_refund_sum = self.invoice.order.payment_refund_sum
+            if payment_refund_sum and (
+                self.invoice.event.settings.invoice_show_payments
+                or self.invoice.order.status == Order.STATUS_PAID
+            ):
+                doc.trade.settlement.monetary_summation.prepaid_total = min(
+                    payment_refund_sum, total
+                )
+                doc.trade.settlement.monetary_summation.due_amount = min(
+                    total - payment_refund_sum, Decimal("0.00")
+                )
         return doc
 
     def _on_first_page(self, canvas: Canvas, doc):

tests/conftest.py

@@ -87,7 +87,7 @@ def order(event, organizer, item):
         sales_channel=event.organizer.sales_channels.get(identifier="web"),
         datetime=datetime(2024, 12, 1, 10, 0, 0, tzinfo=UTC),
         expires=datetime(2024, 12, 10, 10, 0, 0, tzinfo=UTC),
-        total=Decimal("100.00"),
+        total=Decimal("23.00"),
         locale="en",
     )
     OrderPosition.objects.create(

tests/results/paid.xml

@@ -145,7 +145,8 @@ Attendee: Peter</ram:Description>
                 <ram:TaxBasisTotalAmount>19.33</ram:TaxBasisTotalAmount>
                 <ram:TaxTotalAmount currencyID="EUR">3.67</ram:TaxTotalAmount>
                 <ram:GrandTotalAmount>23.00</ram:GrandTotalAmount>
-                <ram:DuePayableAmount>23.00</ram:DuePayableAmount>
+                <ram:TotalPrepaidAmount>23.00</ram:TotalPrepaidAmount>
+                <ram:DuePayableAmount>0.00</ram:DuePayableAmount>
             </ram:SpecifiedTradeSettlementHeaderMonetarySummation>
         </ram:ApplicableHeaderTradeSettlement>
     </rsm:SupplyChainTradeTransaction>