The LORD describes another category of offerings to be given to the priests in verses 15-18, so Good News Translation inserts another paragraph break here. He gives the general principle for these offerings in verse 15, and then the details in verses 16-18. Again he points out that these gifts belong to him in the first place.
Everything that opens the womb of all flesh, whether man or beast, which they offer to the LORD, shall be yours …: For Everything that opens the womb, which refers to the firstborn, see 3.12. Many languages may have a special term that is used to designate the “firstborn,” but only referring to humans, not animals. For whether man or beast, see 3.13. Which they offer to the LORD is literally “which they bring forward to the LORD.” Good News Translation makes explicit the referent for the pronoun they and refers to the LORD in the first person, saying “that the Israelites present to me.” The Hebrew pronouns for yours and you are singular again (see verses 12-13). A possible model for this whole sentence is “Every firstborn son or male animal that the Israelites present to me belongs to you” (similarly Good News Translation).
Nevertheless the first-born of man you shall redeem: Compare 3.46-51. The Hebrew particle rendered nevertheless (ʾak) introduces a strong contrast. The Hebrew verb for redeem is repeated here for emphasis, first as an infinitive absolute and then as a finite verb. Good News Translation expresses the emphasis by rendering this clause as “But you must accept payment to buy back every first-born child,” which is a helpful model since it also indicates that it is the Israelite who presents his firstborn child who pays, not the priest. Another way to make this clear is to translate “but you shall have the first-born of man redeemed” (New Jewish Publication Society’s Tanakh; similarly Traduction œcuménique de la Bible, De Nieuwe Bijbelvertaling) or “but you must allow the redemption of every firstborn son.”
And the firstling of unclean beasts you shall redeem: Firstling renders the same Hebrew word as first-born. In English it is more natural to say “first-born” (Good News Translation) here also. Unclean beasts refers to ritually impure animals, such as donkeys. This clause may be rendered “and you must accept payment to buy back every firstborn [male] animal that is ritually unclean” (similarly Good News Translation).
This verse does not yet specify how much should be paid, but in some languages the following may be a helpful model for you shall redeem: “you must accept an appropriate amount of money for” (similarly Die Bibel im heutigen Deutsch). Firstborn sons belong to God but are not acceptable for sacrifice because their life is sacred, while the firstborn males of unclean animals are not acceptable because unclean animals cannot be used in the cult (so Alter, page 775). The first-born of man and the firstling of unclean beasts are not limited to males here, but this restriction seems to be implied by texts such as 3.40-41 and Exo 13.12-13; 34.19-20; Lev 27.6; Deut 15.19.
Quoted with permission from de Regt, Lénart J. and Wendland, Ernst R. A Handbook on Numbers. (UBS Helps for Translators). Miami: UBS, 2016. For this and other handbooks for translators see here .