... But my problem is that I need one or two things that look completely straightforward to be the first couple of bargains. If the demon asks the wizard to buy a random girl an ice cream in exchange for valuable information, the wizard is going to go "you're a demon, you don't do nice things, so this must be a spectacularly bad idea." Later, when the wizard trusts the demon, it won't be a problem, but the first, second, and maybe third things the demon asks for have to be the kind of thing where the wizard will go "yeah, I can see why a demon would want that, but it's not hurting any innocents, so why not?"
I like the idea of potions, and other magic items.
Near the end of Small Favor, Dresden asks Eldest Gruff for a donut -- a
mortal donut (no faerie glamour, etc).
I can see some other person bargaining with the demon, demanding a potion or other magic-item "Made by a mortal, not a demon! And made without any form of black magic attached or used in creating the item!" So the demon needs a source of said items. And it will explain frankly, "hey, this other mortal dweeb wants something that
I cannot make, but
you can. I'm just tryin' ta do business here, kind of a middleman."
So the demon wants your wizard to create some small-to-medium magic items, and do so entirely free of black magic.
It's a plausible explanation.
The demon may even be telling the truth about it... sometimes, for some of the items...
Maybe.
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The demon may demand something "en passant" of something your wizard is already doing... say your wizard needs to track down a BadGuy(tm) before the innocent victim gets sacrificed on thane altar, and the demon will tell you where to find the BG(tm) and the Evil Altar of Evilness... and the cost will be: you need to bring back some OTHER object you'll find there, and give it to the demon (like when Dresden handed Mavra the
Word of Kemmler, at the end of
Dead Beat).