The method listed here by BettyJaneB does not work.

It will only work if there is an outstanding bill from the vendor that hasn't been paid yet.  That is what the screen shots appear to be showing, an outstanding bill that you click on, then use the credit (created by your deposit) to pay for it.  

Of course, in the real world, this is never going to happen because your vendor is not going to refund you for a bill you have not yet paid.   

I have a vendor who is paid by check on delivery.   We returned an item to that vendor and they cut us a check.   I have deposited the check and I have done a credit memo for the returned item, but I can find no way to link the two.       Can anyone help?  QB 2021 Desktop.   Thanks. -Brett