Only your buyer can cancel / close the return request.

Ask them to do this,  bearing in mind it relies on a lot of trust from your buyer  for you, as they may know,  that you cannot be enforced to make that partial refund, and once a case is closed another for the same item cannot be opened.

In short,  if the case remains open,  you have to accept and fully refund,  a partial refund can only be made if the buyer closes the case early.

@sunflowerhangerco