WHAT IF: He's a multi billionaire and if you return it, it would rekindle his faith in the good of humanity and reward you with like, a farrari or something....OR he could invite you into his life, become like a friend of yours...you would hang out with him poolside at his mansion, go on lavish trips with him on his private jet, courtside seats at the lakers game etc, shows up at your school to pick you up in his helicopter....but eventually you learn that he has a dark side...he starts telling you things....dark things...like maybe he killed someone on a trip to Mexico years back when he was younger...or maybe he hints that he killed his father for the inheritance towards the end of his fathers life....and you like the guy...he's lonely, he's dark, he has a worldview that is very similar to your own...a kind of loner, individualist mentality...but the more horrible things he tells you about his life the more freaked out you start to get...and you try to separate yourself from him, get some distance...you realize that as much alike as you are, your disgusted by him, and by the part of yourself that's like him...so you try to break away from him and get some distance...and at first he doesn't really notice...maybe you say you can't go to the lakers game this weekend, or maybe you say you can't go on that trip to Washington to meet Obama...and you keep distancing yourself, ignoring his calls...and eventually he does notice...and he doesn't like it. He starts showing up unannounced at your house, your school, etc. He accuses you of ignoring him and you guys get into a big fight which ends with him saying something like "if you ever tell anyone the things I told you, about what I did...it won't be pretty" ....then weird stuff starts happening...the scholarship he helped you get goes away, he buys your parents company and fires them....bad things start happening to your friends...and it just gets worse from there....
Or maybe your the psychopath and you use his wallet to mess with HIM. Steal his identity, stalk him, do horrible stuff to him because you get a power rush out of it that you eventually become addicted to...and it just escalates from a prank among you and your friends, to a real obsession, a duel between you and him, with the stakes getting higher and higher...until someone finally gets hurt...
or you could keep the cash, and mail him back the I.D. and wallet in an envelope, either way. You sound like a kid so if you want my honest opinion, the cash isn't what most people would be angry about, its all the personal stuff...especially drivers license, which is a serious pain to replace. You dont want to be responsible for someone waiting 8 hours at the DMV to get a new license do you? And having to cancel all there credit cards etc? Keep the cash, because finders keepers, but send the wallet back though, seriously, otherwise your putting him through a serious hassle thats totally unfair.