Nope , it's usually not available for Private sellers to offer partial refunds when a case is opened , that demands a full refund.
Does your buyer now wish to keep the item and accept a partial refund?
Then, they must first close the case and you can then make that partial refund. If this is not the case, and a return for item not as described has been opened, you have to fully refund through those case details, and within 3 days, or the buyer can escalate on day 4, eBay will force the refund and may tell the buyer they do not need to return the item.
@natbitetools