Hi ..There is nothing more you can do. Did the buyer pay and you refunded. From a seller's perspective , filing a cancellation is for one reason only , the recovery of eBay fees. Should never be done without some signal from buyer that they are willing to accept.
Buyers can be sceptics in such cases , wondering if you are being truthful or found a buyer elsewhere or didn't get the price you wanted etc. Must be handled with TLC.
Buyer's denying suggests that they are not happy and may result in poor feedback , unfortunately.