Numbers 3:13

3:13 because all the firstborn are mine. When I destroyed ¹  all the firstborn in the land of Egypt, I set apart for myself all the firstborn in Israel, both man and beast. They belong to me. I am the Lord.” ² 

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, ³  and the firstborn males of unclean animals you must redeem.

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¹ tn The form הַכֹּתִי (hakkoti) is the Hiphil infinitive construct of the verb נָכָה (nakhah, “to strike, smite, attack”). Here, after the idiomatic “in the day of,” the form functions in an adverbial clause of time – “when I destroyed.”

² sn In the Exodus event of the Passover night the principle of substitution was presented. The firstborn child was redeemed by the blood of the Lamb and so belonged to God, but then God chose the Levites to serve in the place of the firstborn. The ritual of consecrating the firstborn son to the Lord was nevertheless carried out, even with Jesus, the firstborn son of Mary (Luke 2:22-23).

³ 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).