Demon idea: they're egregore-embodiments of ruthlessness and Doing What It Takes, and they're pretty much metaphysical loan sharks. "They never outright lie" is a common belief they do their best to spread.
I like the idea, but I don’t think it fits this story- the magic system, whatever it is, is more about drawing on and extracting power than it is about negotiating the terms of deals. You can be a powerful wizard without any social skills whatsoever- it’s more about navigating the arcane rules of the interface than it is about dealing with demons as people. I’m thinking demons are generally more animalistic, not machiavellian schemers with a powerful bargaining position- but that’s a generalization, there’s lots of different types and the way you have to handle each depends on the sub-school of magic you’re dealing with.
(The ruthless loan shark types are probably represented somewhere- probably higher-level types, not usually employed unless someone needs their very specific services and they’re out of safer options.)