I heard that there are Mitzvos that forbid certain behaviors toward fellow Jews but allow or prescribe those very behaviors toward gentiles.
What are some examples of such Mitzvos?
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Lending with interest for one: "You may not take interest from you brother, neither interest on (a) money (loan) nor interest on (a) food (loan); You may take interest from the foreigner..." Deutoronomy 23:20-21
A very simple rule of thumb for all of those rules.
Anything a gentile is not forbidden in the seven Noachite laws from doing to a Jew, a Jew is likewise not forbidden from doing to a gentile.
A gentile may charge interest to a Jew. A Jew may charge interest to an non-Jew. The laws of interest are internal among Jews.
A gentile may not deceive a Jew. A Jew may not deceive. The prohibition of deception is universal.
A gentile has no Torah obligation to return the lost item of a Jew that he found. Similarly a Jew has no such Torah obligation towards a gentile.
The above is on a Biblical level. Due to the behavior of non-Jews in certain areas things may differ. For example while there will never be a biblical commandment to return the lost item of a non-Jew in a place where non-Jews are very careful to return lost items it would not be so simple for a Jew to keep the lost item of a non-Jew on a rabbinic level either.
Another two examples are:
Considering Chullin 115b - Although the Nevelah prohibition [Devarim 14:21] would allow "perished" livestock to be sold to Ger & Foreigners, the Orlah prohibition [Vayiqra 19:23] would not allow farmers to sell the "withered" fruit from 3-yr-old trees to Foreigners, since [Chullin 115b:8] forbids any (nutritional or monetary) benefit.