Comentario citado sobre Números 3:13

[An Israelite] who buys a fetus of a donkey belonging to a non-Jew or who sells one to him, although this is not permitted, or who forms a partnership with him, or who receives [an animal] from him to look after or who gives [his donkey] to him to look after, is exempt from the [law of the] bekhor, for it says: [“I sanctified to me all the firstborn] in Israel,” (Numbers 3:13) but not in non-Jews. In order for a Jew to be liable to redeem the first-born of a donkey, a non-Jew cannot have any ownership or partial ownership over the donkey. So if a Jew buys a fetus (assumedly he buys a pregnant donkey) from a non-Jew it is exempt from the laws of the first-born, because it was conceived under non-Jewish ownership. Similarly, if he sells the fetus to the non-Jew, it is exempt. The mishnah notes that selling a donkey to a non-Jew is not permitted because it is forbidden to sell a large animal to a non-Jew (see Avodah Zarah 1:6). Nevertheless, if one does sell the donkey, it is exempt. If a Jew and a non-Jew are partners in owning a donkey it is exempt. If a Jew receives a donkey to take care of, and in return he keeps some of the offspring, or if he gives the donkey to the non-Jew to take care of and the non-Jew keeps some of the offspring, in both of these cases, the donkey is exempt because the non-Jew has some level of ownership over the animal. The exemption of non-Jews from these laws is derived from the word “in Israel,” which means that the law of the bekhor applies only to animals fully owned by Israelites.

[An Israelite] who buys a fetus of a cow belonging to a non-Jew or who sells one to him, although this is not permitted, or who forms a partnership with him, or who receives [an animal] from him to look after or who gives [his cow] to him to look after, is exempt from the [law of the] bekhor, for it says: [“I sanctified to Me all the firstborn] in Israel,” (Numbers 3:13) but not in non-Jews. This is the same exact halakhah as was taught in the 1:1. See there for commentary.

weil es heisst. Num. 3, 13.