When you choose to post a return and downcount inventory, if you have entered a GL account in the inventory variance account field in return merchandise setup, then, the system will do a journal entry when you post the return and choose to downcount inventory.  This journal will credit inventory and debit the inventory variance account that you have chosen.

     So, then, when you post the credit voucher in AP, rather than choose inventory for your offset account, you would choose the inventory variance account.  Then, of course, when you want to reconcile your GL inventory to your QOH inventory value, if you had any remaining balance showing in the inventory variance account, you would then know that either you had some returns that you had not received credit for from the vendor, or the vendor gave you less, or more credit than you thought.  Either way, this variance account could be used to explain, and control variances due to returns to vendor.