If the buyer opens a return case for item does not fit, they should be instructed to pay for that fully tracked return postage. On receipt you fully refund everything the buyer initially paid you.
If they open the wrong case that is, one for item not as decribed, then you will have to pay for that fully tracked return postage.
Have they opened a case? Once opened they are all automatic, and refunds for item not as described if that's what has been opened , has to be made in full, or the buyer gets the chance to escalate, eBay step in force the refund, and you gain a defect here, on your account.
@avhking