במדבר כ:ו מדרש: ספרא

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).