To me because you are a new seller, the buyer I think, is playing you for a partial refund, which you do not have to do.
However, if they open a case for item not as described, you will have to accept the case and refund within 3 days. You will want your item back first, so the case will show you how to pay for that fully tracked return postage, and you have 48 hours to refund once your item is back with you.
Do not refund until the item is back.
This way your seller fees are refunded, and your seller performance not affected.
If you feel the buyer has abused eBay's 30 dayss MBG, Money Back Guarantee policy you can appeal the decison for the refund here:
https://www.ebay.co.uk/help/selling/manging-returns-refunds/appeal-eBays-decision-return-missing-ite...
Always block troublesome buyers from returning to your sale here:
https://www.ebay.co.uk/bmgt/buyerrequirements
@asana23