One who buys the fetus of a donkey belonging to a non-Jew, or sells [one] to him, even though it is not permissible, or he enters into a partnership with him, or he receives [a donkey] from him to hold, or gives [one] to him to hold, is exempt from the bechor laws, as it is stated (Numbers 3:13): in Israel —- but not in others.  [The donkeys of] Kohanim and Leviim are exempt because of a kal vachomer: If they exempted the Yisraelim in the Desert, it is only proper that they should exempt their own.