decided to give a refund
But HOW did you give the refund?? If through eBay, the buyer will have been refunded in full.
Did the buyer open an item not received case? Did they send you a cancellation request through eBay? In either case, eBay would have refunded the buyer and you will have received an account credit for your selling fee.
Or did you cancel it yourself, through eBay? If so, the buyer will still have been refunded in full, but depending upon the reason you gave eBay may not have returned your selling fee.
I suspect that what you may be seeing is the refund less the selling fee, which is what will be taken from your account, but is not what the buyer receives.
If you simply refunded the buyer privately in some way it's impossible to help.
If only you would explain things more fully when asking questions it would be so much easier to advise you.
