For a Jedi like Qui-Gon, who's supposed to not be satisfied just being a thug enforcer for a corrupt bureaucracy, the Force should be all the authority he needs. No one's suggesting that Qui-Gon had the time or means to dismantle the entire slave industry of Tatooine, but how about at least trying to save the family that took you in and helped you out when you were stranded on a hostile planet? You took Jar Jar Binks with you, for crying out loud, what's stopping you from locking Watto in the closet while you escape with Anakin and his mom? If the money Qui-Gon offered was worthless, he might as well have just tried mind tricking Watto into giving them the part for free. Morally permissible or not, it's still essentially robbery. And if stealing from Watto isn't a moral problem, why not just steal his slaves and the part outright? The point is that Obi-Wan didn't just immediately start throwing the Force around to get his way. Even when Han was clearly swindling them, Obi-Wan still relied on his personal powers of persuasion over his Jedi powers. Qui-Gon didn't ask Boss Nass for a bongo, he just took it, and when Watto wasn't immediately forthcoming, he again just tried to use his powers to get what he wanted instead of even trying another way. I think Obi-Wan's situation was a lot more desperate than Qui-Gon's.