Bava Batra, Chapter Five, Mishnah Eleven
Mishnah eleven continues to deal with the weighing of produce.
1) Rabban Shimon ben Gamaliel said: When is this so (that one needs to clean out measures)? With regards to liquid measures, but with regards to dry measures it is not necessary.
2) [And a shopkeeper] must let the scales sink down a handbreadth [to the buyers advantage].
a) If he gave him an exact measure, he must give him his overweight, a tenth for liquid measures and a twentieth for dry measures.
3) Where the custom is to measure with small measures they should not measure with large measures and where the custom is to measure with large measures they should not measure with small measures.
4) Where the custom is to smooth down [what is in the measure] they should not heap it up, and [where the custom is] to heap it up, they should not smooth it down.
Section one: Rabbi Shimon ben Gamaliel qualifies the statements in the previous mishnah regarding cleaning out measures. These statements applied to measuring liquids, but measures used for dry goods need not be cleaned out with regular frequency, since the dry goods do not stick to the sides.
Section two: The custom was to let the buyers side of the scale, which contained the goods, sink one handbreadth lower than the sellers side. If, however, one did not follow this custom, then the shopkeeper would be obligated to give the buyer either a tenth or a twentieth more than he purchased, depending on the nature of the produce.
Sections three and four: The last two sections teach that one must follow the customs that are typical in ones place. This is true with regards to the size of the weights as well as the smoothing out or heaping up of the dry goods in the measuring cup. As long as people are consistent in their customs, buyers and sellers will know what they are doing and one will not cheat each other.