Charles Ellicott Commentary Numbers 18:15

"Everything that openeth the womb, of all flesh which they offer unto Jehovah, both of man and beast shall be thine: nevertheless the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem." — Numbers 18:15 (ASV)

Every thing that openeth the matrix ... — (Leviticus 27:26–27.) Five shekels was the redemption-price paid for each of the firstborn who were not redeemed by the Levites (Numbers 3:47).