You need to send them a mutual cancellation request - you can do this by clicking on the More options link next to the item in your sold list, then click resolve a problem. The next screen will have an option that says something along the lines of "I sold and item and want to cancel the transaction", and after confirming that option, you will need to select the most appropriate reason for the cancellation, and send it to the buyer.
They will receive an email and message in eBay about the request and will need to manually agree to the cancellation so that your FVF are credited, so it's a good idea to let them know you'll be sending through the request, and you'd appreciate if they can agree to it at their earliest convenience.
