מדרש אויף במדבר 18:16

(Exod. 30:13:) A HALF SHEKEL. R. Judah and R. Nehemiah differ. R. Judah says: Because (in the matter of the calf) they sinned in the middle of the day, they would give the half shekel. But R. Nehemiah said: Because they sinned for six hours in the day, they would give the half shekel,²²ySheq. 2:4 or 3 (46d); see Tanh., Exod. 9:10. See also below, Exod. 9:13. since it consists of six grammata.²³The Greek word (gramma in the singular) denotes a small weight of 1/24 ounce. R. Judah bar Nehemiah said in the name of R. Johanan ben Zakkay: Because Israel transgressed (rt.: 'BR) the Ten Commandments, they would give ten gerahs.²⁴Cf. Exod. 30:13 = Numb. 3:47: THE SHEKEL IS TWENTY GERAHS. Similarly Lev. 27:25; Numb. 18:16; Ezek. 45:12. R. Berekhyah said in the name of Resh Laqish: You sold Rachel's first-born for twenty <pieces of> silver (according to Gen. 37:28); therefore, in the case of every first-born that you shall have, his redemption shall be five shekels of silver, as stated (in Numb. 3:47): YOU SHALL TAKE FIVE <SHEKELS> EACH.

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).