Order change: Allow to keep price when changing items

src/pretix/base/services/orders.py

@@ -815,12 +815,17 @@ class OrderChangeManager:
         self.notify = notify
         self._invoice_dirty = False
 
-    def change_item(self, position: OrderPosition, item: Item, variation: Optional[ItemVariation]):
+    def change_item(self, position: OrderPosition, item: Item, variation: Optional[ItemVariation], keep_price=False):
         if (not variation and item.has_variations) or (variation and variation.item_id != item.pk):
             raise OrderError(self.error_messages['product_without_variation'])
 
-        price = get_price(item, variation, voucher=position.voucher, subevent=position.subevent,
-                          invoice_address=self._invoice_address)
+        if keep_price:
+            price = TaxedPrice(gross=position.price, net=position.price - position.tax_value,
+                               tax=position.tax_value, rate=position.tax_rate,
+                               name=position.tax_rule.name)
+        else:
+            price = get_price(item, variation, voucher=position.voucher, subevent=position.subevent,
+                              invoice_address=self._invoice_address)
 
         if price is None:  # NOQA
             raise OrderError(self.error_messages['product_invalid'])

src/pretix/control/forms/orders.py

@@ -286,6 +286,7 @@ class OrderPositionChangeForm(forms.Form):
             ('secret', 'Regenerate secret'),
         )
     )
+    change_product_keep_price = forms.BooleanField(required=False)
 
     def __init__(self, *args, **kwargs):
         instance = kwargs.pop('instance')

src/pretix/control/templates/pretixcontrol/order/change.html

@@ -95,6 +95,10 @@
                                 {% trans "Change product to" %}
                                 {% bootstrap_field position.form.itemvar layout='inline' %}
                             </label>
+                            <label class="checkbox">
+                                {{ position.form.change_product_keep_price }}
+                                {% trans "Keep price the same" %}
+                            </label>
                         </div>
                         {% if request.event.has_subevents %}
                             <div class="radio">

src/pretix/control/views/orders.py

@@ -1229,7 +1229,7 @@ class OrderChange(OrderView):
                         variation = ItemVariation.objects.get(pk=varid, item=item)
                     else:
                         variation = None
-                    ocm.change_item(p, item, variation)
+                    ocm.change_item(p, item, variation, keep_price=p.form.cleaned_data['change_product_keep_price'])
                 elif p.form.cleaned_data['operation'] == 'price':
                     ocm.change_price(p, p.form.cleaned_data['price'])
                 elif p.form.cleaned_data['operation'] == 'subevent':