Soul Bonded, #1 by Lori Titus

My soul is in hock, a mystical layaway.

I haven't sold my soul to the devil, but the deal I made is close enough. I have to work for a demon for the next seven years. If I do my job well enough, my contract is void after that time, and I get the part of my soul back that he took away. Unless he decides I'm more valuable to him than that. I've come across worse odds.