IIRC, didn't Mab say he was HER'S, Blood and Bone, Mind and Body?
Well, what if his soul, or some other aspect of his being were to be given or taken by another being? Couldn't that be an out?
I mean, prior to his bargain with Mab, he already belonged to Demonreach. That could be a means for him to be able to leave the service of Mab. If there was some kind of conflict that required that he make a choice of one or the other, DR has an older claim.