Hmm, it seems like theres definitely wiggle room in this deal. He only had said you need to find this sleeping being, rather than bring it to him. I would suggest trying to ask for jacek's full freedom from the devil, then either awakening this being or giving that power to Alexsei, but idk how jacek himself would end up feeling about the loss of demonic magic, or if the devil would be willing to give up one of his favorite playthings.