You could take him to small claims court for the $700 and possibly more (due to the RIs, feeding, etc. Keep receipts and records of everything), but it would cost you money up front to do that. I would absolutely hold him accountable for the full amount agreed upon, instead of you cutting him some slack. I would recommend taking him to court over it, especially if you're not trying to save a friendship (doesn't sound like he was a friend to start with!), if you have the money to do so. Sorry things went wrong here, it sounds like plain bad luck. Good luck.

(PS- I am a newbie to snakes, but I have seen stuff like this countless times with dogs, horses, etc.)