I'm with most of the people here. Adults are considered to be mature enough to stick to their word. He struck a deal, he should have researched before.
500$, even when the law is at your side, is an enough high amount for an angry person to keep buggering you forever. But if you let him go for 50 bucks he won't learn the lesson.
That way, I would keep at least $200 for me, and give him the other 300, but reluctantly. So he will be happy to get at least part of his money back, and the experience will teach him a lesson to be more serious in the future.
Bottom line? He has been told the bike is too expensive, but he is going to leave without 200 bucks. So he will buy a cheaper one that will cost him 200 bucks more. Maybe he would rather buy yours instead?
The other day I won a couple auctions on eBay. I regret bidding too late -because I didn't really need them-, and told the seller how about paying him the fees and he relist the items. The price with shipping included was 40 euros. He told me it was fine for him if I paid him 10 euros. It was too much in my opinion, but I was the winning bidder. So between wasting 10 euros, or paying 30 euros more and getting the parts, I finally stuck to my winning bid and sent the payment.
Raul