Use line item quantity to calculate unit price of Shopify sale line

The total discount is based on the initial quantity of the line item instead of
the current quantity.

Closes #14187

modules/web_shop_shopify/sale.py

@@ -392,9 +392,9 @@ class Line(IdentifierMixin, metaclass=PoolMeta):
         total_discount = sum(
             Decimal(d.amount) for d in line_item.discount_allocations)
         unit_price = ((
-                (Decimal(line_item.price) * (quantity or 1))
+                (Decimal(line_item.price) * (line_item.quantity or 1))
                 - Decimal(total_discount))
-            / (quantity or 1))
+            / (line_item.quantity or 1))
         unit_price = round_price(Tax.reverse_compute(
                 unit_price, line.taxes, sale.sale_date))
         if line.product: