If the buyer has opened a case for item not-as-described then you respond with the offer of a full refund on return of the boots. You are entitled to refuse the return and you are entitled to refuse to respond to the case but if you do either of those things, eBay will assume that you don't want the boots back and will simply force the full refund when the buyer escalates the case.
The resolution centre is set up to put both parties back into their pre-transaction positions because eBay, who were not present at any time during the transaction, has no way of knowing who is correct.
Do not refund outside a case and do not refund until you have the item back.
