You are right he doesn't have a leg to stand on when asking for a partial refund. Partial refunds cannot be enforced and only have the go ahead, when agreed by both seller and buyer.
What you have to accept is if they open a case for item not as described, then you do have to accept the case within 3 days and refund , or issue a paid return tracked label if you want the item back first.
You only refund when the item is back with you, you have 48 hours to do so.
Stick within those timeline and your seller performance is not affected.
@danger_days92