Purchase credits are used for receiving/requesting credits or refunds from purchases (suppliers). They are essentially the same as a purchase order but will apply a negative amount and quantity. The purchase credit process requires that you write off/send back the actual stock items that were the issue, rather than send back the oldest item using a FIFO method. To prevent confusion stock items cannot be added to purchase credits using a product search, you will have to manually type the details of the items. You will then need to do a stock correction to indicate the items are no longer in stock.
If you have raised a credit note for stock you have returned you will need to perform a stock correction to remove the correct quantity and value.