I admit it's kind of weird he doesn't acknowledge the soul coin that could be in your inventory which should be able to be used as contractual tender, and he won't let you just sign away your soul immediately. That said, there's only one encounter in what is clearly going to be a series of repeating encounters. It's real hard to be too judge-y about him.
He might be after something else than simply a soul. In the harpy nest there is
a letter from a warlock who made a deal with a cambion and was sent on some quest. This cambion apparently likes to show up during the night for a chat. There is also a journal, but I've found the descriptions there confusing. I guess you need to know the D&D lore to decipher that.
