Nothing bad, except his soul would eventually be in the hands of something else, whether physically or just metaphorically. You see, that's what people always say, Ahiru. They think- oh what the hell? It's just my soul, right? What can it hurt? And then the next thing you know, the contract's up and someone's facedown in a pool of their own blood, the apocalypse is banging on the door, and someone's soul is on a rack downstairs getting the humanity tortured out of it.

Nothing good comes out of that kind of sacrifice. Just pain of the worst kind. I've seen it. It ain't pretty.