Torah Portion: Mishpatim
Do not hurt the feelings of a foreigner or oppress him (Shemos 22:20).
It is a negative commandment that we not cheat a convert monetarily. That is, that if we should have any business dealing with a convert, it is forbidden to [overcharge or underpay] him, as it is written: Do not hurt the feelings of a foreigner or oppress him (Shemos 22:20). Our Sages teach that the expression, “or oppress” refers to financial mistreatment.
This negative commandment is a separate prohibition with regard to mistreating converts, in addition to the general prohibition that forbids cheating any Jew. The reason why the Torah writes an additional prohibition with regard to a convert is the same as the reasons mentioned above [with regard to verbal mistreatment—that he has no relatives to defend him and that there is concern that he will revert to his old ways].