This is a common practice: the customer enters the store, with his receipt in hand, he goes to get the product "to repay" on the shelves and shows up at the counter with the product and invoice, he pockets the refund and you're done.
To prevent :
- Place the return counter as close to the entrance as possible.
- Review the video footage of the customer's arrival, did he have the product in his hands
- Make sure that a trash can is available, as close as possible after the payment so that the cash register does not become available to fraudsters.