Refund requests

I have a small problem. Imagine this situation: I run a clothing store. A customer buys two T-shirts of the same type. Later, the customer says he only needs one T-shirt and requests a refund for the other one. How should I handle this? I tried to refund just one item from the two identical items, but the system didn’t allow me to do that. Is it possible to process a refund for only one item in such a case?