There's lots of different kinds of slavery out there. It sounds like you're thinking of something like a Janissary, or someone under a magical geas. And he could mark experience by doing dirty jobs for his master, and use that experience to either buy his freedom or become more effective doing whatever he does. I'd totally play that, it sounds fun and disturbing both.
But maybe you should change the name a bit, the word "slave" in the USA tends to refer to our own shameful history and that's really not what you're talking about at all.