In all fairness, taking obligation to afford an item is always an option. After all, what is money but tokens of favour?
that being said, obligation is one of the few resources that the players have pretty good control over that they can’t be forced to take. Obligation is always a result of a consensual decision, even if they aren’t of making it sometimes. XD That being said, I don’t feel being unable to afford something should go down that rabbit hole normally unless the player vouches for it, the NPC is immensely influential and dangerous to force them to take the deal (which I would only use sparingly for the characters who really need to be hated) or it’s the result of some fine or penalty. I believe all those things could be reasonable provided context. I don’t believe anything fits all the boxes, all the time.
I’m less interested in the debating part of things; Qui-Gon is a seasoned negotiator, just the challenge was finding Watto’s Price/kink so to speak, which was winning this race that needed that child. I could believe the entire conversation was about giving this boy to win his own freedom, with the hyperdrive; it succeeded with the caveat of him having to win a rigged game of dice.