The firstborn of every womb, whether man or beast, that is offered to the LORD belongs to you. But you must surely redeem every firstborn son and every firstborn male of unclean animals. You are to pay the redemption price for a month-old male according to your valuation: five shekels of silver, according to the sanctuary shekel, which is twenty gerahs. But you must not redeem the firstborn of an ox, a sheep, or a goat; they are holy. You are to sprinkle their blood on the altar and burn their fat as an offering made by fire, a pleasing aroma to the LORD. And their meat belongs to you, just as the breast and right thigh of the wave offering belong to you.
you are to present to the LORD the firstborn male of every womb. All the firstborn males of your livestock belong to the LORD. You must redeem every firstborn donkey with a lamb, and if you do not redeem it, you are to break its neck. And every firstborn of your sons you must redeem.
Significance
Exodus 13:12-13 establishes the firstborn consecration law: every firstborn male must be set apart for God, with the option to redeem a donkey's firstborn by substituting a lamb. Numbers 18:15-18 assigns this system to the Aaronic priests and adds critical details: human firstborn must be redeemed at five shekels (v. 16), but firstborn of clean animals (ox, sheep, goat) may not be redeemed -- their blood is sprinkled on the altar and their fat burned as a רֵיחַ נִיחֹחַ (reyach nichoach, "pleasing aroma"). Numbers thus transforms the Exodus principle into a functioning priestly revenue system while maintaining the theological foundation: Israel's firstborn belong to God because He spared them at the Passover. The manner of offering these clean firstborn animals--blood sprinkled, fat burned--mirrors the peace-offering pattern, tying firstborn consecration to Israel's broader sacrificial system.
