Very odd. So it definitely sounds like someone inexperienced who thought they could take advantage of you by just saying they got the wrong item. As reputable sellers we know that stuff like this doesn't just happen. If the game does actually come back as it was, give him the cash back and just be glad it wasn't worse. If it's opened, meh, some negative feedback is probably warranted but no big deal. If nothing shows up or something else comes, I'd think there are steps in the return process that you can take as a seller to protect yourself just as a buyer can when purchasing. Keep an eye on how long the case has. I've only had one problem with not receiving the correct item and it was cheap enough that it worked out in my favor. Seller wanted me to pay to return a $2 keychain, and I said "No way, that'd be more than twice what I paid for it, send me the right one!". He got some bad feedback for his negligence and had to make a new eBay account to restore his pristine feedback lol. Policies have changed a lot since then though.
I bought a fraudulent Club Nintendo code and have no way to say I didn't receive my item since the seller marked it as shipped. I can't report the item as significantly not as described since I haven't paid. So I'm stuck with eBay asking for cash on something I didn't get, and it stays stuck in my cart in the mobile app for some reason lol. Oh well, don't know if there's anything else I can do. If I get an unpaid item strike it will go away eventually. I rarely have problems with eBay.