The devil would have an infinite amount of money.

So I'd want payments every month of a set ammount of money, done in a binding contract, until the day I naturally die, and then the money will be able to stay far along to my children and grandchildren.
Naturally die also means, the devil would be unable to basically kill me off, my life would have to play out as if I never sold my soul, but instead of being poor I'd be rich.
That'd be about it to me.