Midrash for Numbers 18:16

7) (Vayikra 27:24) "In the Yovel year the field shall return to the one from whom it was bought": I might think, to the Temple treasurer, from whom it was bought (by the last purchaser); it is, therefore, written "to the one who has the holding in the land (i.e., the original owner). (In that case let it be written [only] "to the one who has the holding in the land." Why state "to the one from whom it was bought'? (For I might think that) a field which went out to the Cohanim on the Yovel and was sold by the Cohein (who acquired it), and was consecrated by the buyer — I might think that when the second Yovel arrived, it reverted to the original owner (whose field of holding it was before he consecrated it); it is, therefore, written "to the one from whom it was bought" (namely, the Cohein who sold it, [as opposed to the original owner, who consecrated it]). (Vayikra 27:25) ("And all of your valuations (concerning which it is written "shekalim") shall be according to the shekel of the sanctuary; twenty gerah shall the shekel be.") "And all of your shekels shall be according to the shekel of the sanctuary": There is no valuation less than a sela (the same as a shekel). "according to the shekel of the sanctuary": What is the intent of this? Because it is written (Vayikra 27:27) "and he shall redeem," I might think, with servants, deeds, and land; it is, therefore written "with the shekel of the sanctuary." This tells me only of the shekel of selaim of the sanctuary. Whence do I derive for inclusion anything that is (of monetary value and is) movable? From "and he shall redeem." If so, why is it written "with the shekel of the sanctuary"? To exclude servants, deeds, and land.

8) "twenty gerah shall the shekel be": Whence is it derived that if he wishes to increase (the value of the shekel), he may do so? From "shall be" (connoting the possibility of a change). I might then think that if he wishes to decrease (the value) he may do so. It is, therefore, written (Bamidbar 20:6) "It is (twenty gerah"), (i.e., at least twenty gerah).

A firstborn human must be redeemed when thirty days old;¹³Firstborn boys are redeemed through the pidyon ha-ben ceremony at the end of thirty days. As indicated above, those born by Caesarean section are exempt. if it survives less than this it is considered a premature child and is exempt from this regulation. The firstborn beast must be redeemed on the eighth day; if it survives less than this it is considered a premature birth. In reference to a human, it is written: And their redemption money—from a month old shalt thou redeem them (Num. 18:16), while in regard to beasts, it is written: But from the eighth day and henceforth it may be accepted (Lev. 22:27). That is, after one is able to lead it to the Temple, since it is said: And thither you shall bring your burnt offerings (Deut. 12:6). In the case of the firstborn of your flock and herds, Scripture says: Thou shalt redeem. This implies that one may redeem the offering from the priest whenever (he wishes). (But if that is so)¹⁴Etz Joseph omits the parenthesized words. Why does the Scripture say: Thou shalt sanctify to the Lord? So that you receive a reward for so doing. But even if you should not sanctify it, it is consecrated, nevertheless, to the Lord, since as Scripture says: It is mine. Why then does Scripture decree Thou shalt sanctify it? In order that you may be rewarded for doing so (voluntarily).