Hi Sanovnik
This is one of the many purchasing/receiving functions you cannot do easily using Adagio Inventory.
Purchase (and returning) inventory is much better handled using Adagio Purchase Orders than directly in Adagio Inventory. In that module you can process returns and have them automatically flow to the vendor's account as a credit.
The only way to do what you want without the PO module is to do an adjustment in Inventory (make a note of the GL adjustment account), and then to enter a credit note in AP, for the same amount and using the same GL adjustment account.
Regards
Steve