What I suggest as you have received the item back not in the same condition as sent out is to contact eBay Customer Services and explain your findings.
Unfortunately with a weekend coming next I'm not sure if Dublin will answer first thing when the lines open at 9 a.m. on weekends and they do usually at 8 a.m. lines opening on weekdays, it's preferable to speak with Dublin as you do have a case for not fully refunding the buyer.
Contact them they may allow a partial refund to go through or advise for a full refund, which is the norm to be made.
It's a difficult one. Returns stipulate the item has to be returned in the same condition as sent out for a full refund. Refund in full straight away , we've recently been advised, this shows you Agree to a refund.
Proving intentional damage may be diffcult and as you have a short window to refund, the buyer can escalate after 48 hours after you received your item you have to act fast.