Did a trade for a computer on craigslist, trader wants his comp back?

I did a trade on craigslist for a macbook pro "traded my dell xps m1730" the guy looked over the computer for 45 minutes. and walked away happy. He called me later and said that there was a different graphics card inside that was in the email. I had a typo for an 8800 gt.... he wanted a 8800 gtx. Two different cards. It was an unintentional mistake. Either way, he should have checked out the comp when he did the trade? is there anthing he can do legally? I do not wish to trade back, or help him buy the replacement card for 1300 bucks.