Everything that opens the womb of all flesh, whether man or beast, which they offer to the LORD, shall be yours. Nevertheless, the firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem.
15. Every thing that openeth the matrix. The same thing is now ordained as to the first-born, viz., that the priests should have them also 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.
The priest's service is called a warfare; and who goes a warfare at his own charges? As they were well employed, so they were well provided for, and well paid. None shall serve God for nought. All believers are spiritual priests, and God has promised to take care of them; they shall dwell in the land, and verily they shall be fed, and shall not want any good thing.
Commenting on Numbers 18:8-19
And those that are to be redeemed,.... Which is to be understood not of unclean beasts last mentioned, for they were redeemed with lambs and not with money; and after they were eight days and not a month old, as follows, Exo 13:13; but of the firstborn of men: from a month old shalt thou redeem, according to thine estimation: that is, take the redemption...