Negative Mitzvah 253
It is forbidden to cheat a convert in business
Exodus 22:20 "We are forbidden to cheat a convert in business deals"
In any case involving business deals, buying or selling, it is forbidden to cheat the convert.
The Torah gives us a special commandment in regard to the Ger in order to make sure that we treat any convert to Judaism just as we would treat any other Jew.