[Think of it a lot like a loan with your soul as collateral. Only it also served as payment when you died.]

Soul manipulation is a very complicated process. Very likely, the magic stored in the reliquary originally came from the demon's own essence. The essence returned to the demon after it was broken in addition to yanking your Rasputin back to the demon's realm.

Though that's neither here nor there. The point is that he's going to be paying for that Deal for a long, long time.