Issuing a refund

Description

I received £25 from someone on the 11th September. I decided to refund the amount directly from my paypal because I didn’t have the item he wanted. For some reason, on the 17th, I get a note saying the refund couldn’t go through because I didn’t have the funds, although I did, because he had just sent me the money and instantly I refunded. He has now said he hasn’t received his refund, but I now don’t have the £25 in my account either. So where has this money disappeared to, and how do I issue his refund?