How to deal with Vendor Credits on Prepaid Purchase Orders

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We regularly import products from overseas and prepay 100% of those purchase orders. Sometimes we end up changing items, leaving us with a credit with that vendor. They typically apply it as a credit on future purchase orders. What is the best practice for entering these credits? Sometimes the credits are very hard to track, and it doesn't really make sense for us to spend so much time looking over previous orders to figure it out. If there was a discount field, I would use it. We once added it to freight, but obviously this was a bad idea for when landing costs are allocated. We would appreciate any suggestions! Thanks