So, Qui-Gon had a bunch of Republic Credits with him on Tatooine. He was just going to buy the parts he needed for their ship straight from Watto. When Watto said Republic Credits were no good to him, Qui-Gon attempted to use the force to trick him to accept it as payment. We find out mind tricks don't work on his species. Nobody else has the hyperdrive he needs, or so Watto says. Qui-Gon seems to just take Watto's word for it. That's right: Qui-Gon accepts that Watto has the only hyperdrive on the entire planet that will fix their ship, and he just happen to walk into his shop on the first try. However, even if Watto was telling the truth, there are other shop owners who could be mind-tricked into accepting the Republic Credits for other valuable goods. Qui-Gon could buy those goods and trade them to Watto, or sell them for local currency that Watto could accept. Or, he could trade the credits with someone who is on their way to Republic space and would need to change their money anyway. At least they are consistent about what kinds of species are immune to mind tricks: the ones that are holding people as slaves!