Fix error when marking an expired order as paid

src/pretix/base/models/orders.py

@@ -208,7 +208,7 @@ class Order(LockModel, LoggedModel):
     def pending_sum(self):
         total = self.total
         if self.status in (Order.STATUS_REFUNDED, Order.STATUS_CANCELED):
-            total = 0
+            total = Decimal('0.00')
         payment_sum = self.payments.filter(
             state__in=(OrderPayment.PAYMENT_STATE_CONFIRMED, OrderPayment.PAYMENT_STATE_REFUNDED)
         ).aggregate(s=Sum('amount'))['s'] or Decimal('0.00')

src/pretix/control/forms/orders.py

@@ -1,3 +1,5 @@
+from decimal import Decimal
+
 from django import forms
 from django.conf import settings
 from django.core.exceptions import ValidationError
@@ -86,7 +88,7 @@ class MarkPaidForm(ConfirmPaymentForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         change_decimal_field(self.fields['amount'], self.instance.event.currency)
-        self.fields['amount'].initial = max(0, self.instance.pending_sum)
+        self.fields['amount'].initial = max(Decimal('0.00'), self.instance.pending_sum)
 
 
 class ExporterForm(forms.Form):

src/pretix/helpers/money.py

@@ -19,6 +19,8 @@ class DecimalTextInput(TextInput):
             return None
         if isinstance(value, str):
             return value
+        if not isinstance(value, Decimal):
+            value = Decimal(value)
         return formats.localize_input(value.quantize(Decimal('1') / 10 ** self.places))