DEUT1465 The Rabbis taught: Whence is it derived that one should not be precise [in weighing] in a place where an "extra" is generally given [to the customer], and that one should not give an "extra" in a place where it is customary to be precise? From: "a whole stone." And whence is it derived that if one said, in a place where it is customary to give an extra: I shall be precise, and deduct [the amount of the "extra"] from the price; or, in a place where it is customary to be precise: I shall give him an "extra" and add something to the price -- whence is it derived that he is not heeded? From: "A whole and just stone" (Bava Bathra 89b)