Incorrect credit payment for second return if order items have item and order promotion

Description

Steps to reproduce

1. Go to a Quicksilver site.
2. Add item "SKU-44477844" with quantity = 12 to cart then checkout.
3. Complete the order.
4. Create a return for one or two items above.
5. Create a second return for one item.
6. Complete the second return.
7. View the payment tab.

Expected: Credit payment = return total
Actual: Credit payment is less than return total