John Calvin 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. The same thing is now ordained concerning the first-born, namely, that the priests should also have them for themselves; though at the same time a distinction is inserted, that the first-born of man should be redeemed. With regard to unclean beasts, the owners were free either to redeem or to kill them.

But, since this matter is not formally treated here, God only briefly declares that He gives to the priests whatever profit may be made from the first-born. The command that the first-born should be redeemed according to the estimation of the priests does not mean that the priests themselves should prescribe the value, as if they had the authority to do so. Instead, it refers to the estimation by which they were bound according to God’s command, as we saw elsewhere. This may be readily gathered from the context, because the price which God Himself had fixed is specified shortly thereafter.

Concerning the first-born of clean animals, another law is given, namely, that they should be killed at the altar and their fat burned, while the flesh was to belong to the priests, like the breast and the right shoulder of the burnt-offerings.

But, to prevent any of the Levites or of the people—since men are always eager for innovation—from ever attempting to violate this decree, all controversy is removed for future ages when God declares that what He gave to the priests He would never take away from them. First, He uses the word edict or decree,²¹⁰ which others translate “statute:” and then adds the title “covenant,”²¹¹ so that its observance may be more sacred and less exposed to contentions and quarrels. For nothing could be more improper than for the priests to dispute regarding their rights and privileges.

God, then, signifies that He Himself will be outraged if anyone should trouble the priests. By the word “salt,” perpetuity is metaphorically expressed. In this, however, God appears to allude to the sacrifices, which it was not lawful to offer unless seasoned with salt, so that the Israelites might learn that by earthly and corruptible things, something greater was designated. For we know that salted meats do not so easily become corrupt. In a word, this metaphor implies inviolable stability.

²¹⁰ חק from from חקק describere, decernere, statuere. — Taylor’s Concordance.. — Taylor’s Concordance.

²¹¹ Addition Fr. “voire, Paction de sel."