Numbers 18:15

18:15 The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, ¹  and the firstborn males of unclean animals you must redeem.

Exodus 13:13

13:13 Every firstling ²  of a donkey you must redeem ³  with a lamb, and if you do not redeem it, then you must break its neck. ⁴  Every firstborn of ⁵  your sons you must redeem.

[18:15]  ¹ tn The construction uses the infinitive absolute and the imperfect tense of the verb “to redeem” in order to stress the point – they were to be redeemed. N. H. Snaith suggests that the verb means to get by payment what was not originally yours, whereas the other root גָאַל (ga’al) means to get back what was originally yours (Leviticus and Numbers [NCB], 268).

[13:13]  ² tn Heb “and every opener [of a womb].”

[13:13]  ³ tn The verb תִּפְדֶּה (tifdeh), the instructional imperfect, refers to the idea of redemption by paying a cost. This word is used regularly of redeeming a person, or an animal, from death or servitude (S. R. Driver, Exodus, 109).

[13:13]  ⁴ tn The conditional clause uses an imperfect tense; this is followed by a perfect tense with the vav consecutive providing the obligation or instruction. The owner might not redeem the donkey, but if he did not, he could not keep it, he had to kill it by breaking its neck (so either a lamb for it, or the donkey itself). The donkey could not be killed by shedding blood because that would make it a sacrifice, and that was not possible with this kind of animal. See G. Brin, “The Firstling of Unclean Animals,” JQR 68 (1977): 1-15.

[13:13]  ⁵ tn Heb “and every firstborn of man among your sons.” The addition of “man” is clearly meant to distinguish firstborn humans from animals.