Did the buyer open a case for item not as describe... item faulty? Then, as cases are all automated , no human contact, you cannot challenge the decision at this point , and need to refund through those case details.
Then, you appeal the decision for the refund with eBay. It's not easy, eBay do not see the item in question, you need to stress the item was working when tested on return, and this buyer has abused eBay's MBG policy to receive a refund.
This explains how to appeal:
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@freakonaleash89