There are several other references of the same sort as the "You're buying" line. But that could very well be because Gene would not have been on set for every single episode nor review every single line in every script. I cannot remember any TOS or early TNG episodes where money was a central concept to the plot (except when it is portrayed as the root of all evil because the Ferengi crave it). Gene is also quoted by very indirectly by friends of other producers as having said that there is no money in the Federation.