Chullin 12:5 - Bechoros 1:1

מצוה שהיא כאיסר – that there is no loss of money in it, other than a small amount.

הלוקח עובר חמורו של נכרי – when it is in the womb of its mother and it is a firstling (i.e., male or female). And for this reason, it (i.e., the Mishnah) took [the words], “the fetus of his donkey,” for there is no holy unclean animal with regard to its first-born other than the firstborn of a donkey alone, but since the words are few of the firstborn of the donkey, he decided and permitted it. But all of these other chapters speak of a first born of a pure animal.

אף על פי שאינו רשאי – to sell to a heathen a large animal, because of work on Shabbat that would be performed by it (see Tractate Avodah Zarah, Chapter 1, Mishnah 6 – where the Halakha is established that it is forbidden to sell a heathen a large animal – originally owned by an Israelite).

והמשתתף לו – even if the portion of the heathen is substantial and known, as for example, the [yet unborn] firstborn’s arm or leg, or that of its mother. Any time that if they would sever from it that limb, the animal remains with a blemish – this is the partnership of the heathen and it is exempt [from redemption].

והמקבל ממנו – that the female donkey belongs to the heathen that an Israelite receives and takes care of it in order that they will divide its offspring, but the body of the animal belongs to the heathen.

פטור מן הבכורה – that fetus that he (i.e., the Israelite) purchased from the heathen or that he sold to the heathen is not holy, for since the heathen has a portion in it or in its mother, as it states (Numbers 3:13): “I consecrated every first-born in Israel, [man and beast, to Myself, to be Mine],” but not of the heathen.

אם פטרו את של ישראל במדבר – the Levites did not exempt the animals of the Israel in the wilderness, that the first-born of the cattle, [the] cattle [of the Levites] replaced [and redeemed] it, as it is written (Numbers 3: 45): “and the cattle of the Levites in place of their cattle,” but rather, this is how it should be read: The Levities released/served as ransom the holiness of the first born of the Israelites in the wilderness, as it is written (Numbers 3:45): “Take the Levites in place of all the first-born among the Israelite people, [and the cattle of the Levites in place of their cattle; and the Levites shall be Mine, the LORD’s],” but if the sanctity of the Levites released/ransomed the sanctity of the Israelites, it is a law that they would release/ransom themselves. But after we learned that the sanctity of the first-born was released/ransomed from the livestock of the Levities, we learned that the released/ransomed of their donkeys are exempt from [the rules governing] the first-born, as it is written (Numbers 18:15): “but you shall have the first-born of man redeemed, and you shall also have the firstling of impure animals redeemed,” all that the first-born of man has, the first-born of impure animals have, and all that the first-born of man doesn’t have, is not found in the first-born of impure animals. But from the first born of pure animals, the Priests and Levites were not exempt, as we learn ahead.