All of the Pentateuch’s sources agree that firstborn human males, firstborn male animals, and the firstfruits of the harvest are to be consecrated to Yahweh.
“Consecrate every firstborn for me [Yahweh]. The first birth of every womb of the children of Israel, of a human and of an animal, is mine!” (Ex 13:2)
The “historicised” rational given for this archaic rite, which most likely reflects an older period of human sacrifice (see #145), is the Passover narrative on the eve of Israel’s exodus from Egypt (see #190-110). There we learn that Yahweh decrees all firstborns—humans and animals—of both the Egyptians AND the Israelites (which most readers miss) to death! However, Yahweh redeems the Israelite firstborns through the pascal blood rite, giving way to the sacrificial theology pronounced in all the Pentateuch’s sources: that all firstborns are Yahweh’s. However, human male firstborns are redeemable—like a coupon—by the Levites, and impure firstborn animals—animals other than ox, sheep, and goat—are redeemable as well. In other words these “ransoms” allow that human firstborn males are not thrown onto the pyre as whole burnt-offerings to Yahweh, and the same for impure firstborn animals.
The problem is that the sources of the Torah give different and contradictory responses concerning the redemption of impure animals—i.e., non-sacrificial animals—and particularly when speaking of a firstling ass.
The two older sources, the Elohist (Ex 13:13) and Yahwist (Ex 34:20), state that a firstling ass must be redeemed with a lamb.
“But every firstling ass you shall redeem with a lamb; if you do not redeem it, you must break its neck”
That is a farmer who owns a firstborn ass can elect, instead of consecrating it to Yahweh for sacrifice, to redeem it by offering a lamb instead, or kill it!
The later Priestly sources, however, speaking more broadly of all impure animals in general, to which category asses belong, has this to say:
And if it is of the impure animals, then he shall redeem it at your [the Aaronid priest’s] appraisal, and he shall add to it a fifth of it.” (Lev 27:27)
“And you shall redeem the firstborn of an impure animal. And their redemption price: from one month old you shall redeem it at your appraisal, 5 shekels of silver by the shekel of the Holy.” (Num 18:15b-16)