Currently, you don't have a way to link a cash receipt (specifically a refund from a vendor) to the vendor record or AP invoice. The only solution I see in your help section is to create a "dummy" invoice and credit memo in the vendor record, apply the credit memo, and select "do not post" for both. It is not ideal to create a fake invoice, and it would be nice to have a way to link AP refunds to vendors and the specific invoice they relate to. Otherwise, it creates a lot of work to hunt down where the cash receipt came from, and which invoice was fully or partially refunded.