Halakhah do Wyjścia 34:20

That is that He commanded us that we should redeem the [firstborn] donkey specifically with a sheep, and it should not be redeemed with anything else. And one should give that sheep to a priest. And that is His, may He be blessed, saying, "But the firstling of a donkey you shall redeem with a sheep" (Exodus 34:20). And the regulations of this commandment have already been explained in Tractate Bekhorot. And the Levites are also not obligated in this commandment. (See Parashat Ki Tissa; Mishneh Torah, Firstlings.)

That is that He commanded us to behead the [firstborn] donkey if we do not redeem it. And that is His, may He be blessed, saying, "if you do not redeem it, you must behead it" (Exodus 34:20). And the regulations of this commandment have already been explained in Tractate Bekhorot. And the questioner may challenge and say, "But why did you count its redemption and its beheading as two commandments; and why did you not count them as one commandment, such that its beheading be from the laws of the commandment - like [you explained] in the seventh principle?" Behold God knows that the analogy requires this, were it not that we found with our Rabbis a language that indicates that they are two commandments. And that is their saying, "The commandment of redemption precedes the commandment of beheading; the commandment of levirate marriage precedes the commandment of chalitzah" (Bekhorot 13a). And just like a levirate wife is suited for either levirate marriage or chalitzah; and levirate marriage is one commandment and chalitzah is [another] commandment - so too, is a [firstborn] donkey suited for either redemption or beheading; and each of them is a commandment, as we have said. (See Parashat Ki Tissa; Mishneh Torah, Firstlings.)