Question

A customer is attempting to return merchandise that you believe was purchased elsewhere

Answer 1

In such cases, the person who wants to change it has to show a receipt of the purchase. If the person does not have a receipt and still persists you can call the manager and the manager can solve it. If the manager is not there, you can say that there's nothing that you can do and that the person can return at a later time when the manager might be there. You can also point them to your customer service.