פירוש על בכורות 1:5

ולא בחיה – a deer or a ram.

Introduction This mishnah provides limits as to what type of animal can be used to redeem the first-born of a donkey.

ולא בשחוטה – a slaughtered young sheep/goat.

We do not redeem with a calf, a wild animal, an animal slaughtered, a terefah, kilayim (a mixed or a koy. The redemption must be done with a lamb, one born of a sheep or a goat. A calf, born of a cow or a wild animal cannot be used. The lamb must be alive when used to redeem the donkey. It cannot be either slaughtered or a terefah (died without being properly slaughtered). Kilayim is a mixed breed. Here it refers to an animal born of a mix between a sheep and a goat. According to the first opinion, a kilayim is not a lamb. A koy is an animal which the rabbis couldn’t identify as either a domesticated or a wild animal (see Hullin 6:1). Since its status is not known, it cannot be used to redeem the first-born donkey.

ולא בטריפה – as for example, that its feet were cut off from the knee and above.

Rabbi Eliezer permits [redemption] with kilayim because it is a lamb. But he forbids with a koy, because its nature is doubtful. Rabbi Eliezer disagrees with regard to the kilayim. Since both a sheep and a goat have offspring that are called “lambs (seh)” this animal can be used. He agrees, however, that a koy cannot be used.

כלאים – a he-goat that comes upon a ewe.

If he gave [the first-born of a donkey] itself to the priest, the latter may not keep it, until he sets aside a lamb in its place. He cannot simply give the first-born donkey to the priest. If he does so, the priest must return it to him, until the Israelite sets aside a lamb to redeem the donkey. At this point, the priest may keep the donkey, if the Israelite wants him to have it.

מפני שהוא שה – whether a ewe or whether a goat, they are called a young goat/young sheep. But the Halakha is not according to Rabbi Eliezer.

כוי – it is a creature of its own, doubtfully wildlife and doubtfully an animal.

נתנו לכהן- the actual first-born donkey.

עד שיפריש שה תחתיו- and this for himself. But afterwards he can use the first-born donkey. But because the Kohanim were suspected in this matter of not separating a young sheep in its place, therefore, an Israelite who gives a first-born donkey to a Kohen does not move from there until the Kohen redeems it in his presence.