The right thing to do? Return it.
The legal thing to do? Any lawyer types around? I don't know y'alls laws there.
The greedy (& not necessarily bad) thing to do? Keep it.
How much would it help you to keep it? Would it get you a car, a semester of school? Something super major? Or major to you? Then keep it & go on about your journey. You'll lose him as a client of course, but you know that.
Are you a believer in karma? (you don't have to be, I'm just trying to cover all bases) Perhaps if you keep it, this'll come back to bite you in the ass, IDK.
Myself, it'd bug me (ME) from a moral standpoint (keeping it I mean), but my integrity gets in the way of things a lot *grumbles*.
I do like the idea of keeping a bit of it for having to listen to him.