פירוש על בבא בתרא 5:11

וחייב להכריע טפח – from the weight a liter and beyond, but less than a liter, he doesn’t need the customary additional weight in retail of a handbreadth (see Tractate Bava Batra 89a).

Rabban Shimon ben Gamaliel said: “When is this so (that one needs to clean out? With regards to liquid measures, but with regards to dry measures it is not necessary.
[And a shopkeeper] must let the scales sink down a handbreadth [to the buyer’s advantage]. If he gave him an exact measure, he must give him his overweight, a tenth for liquid measures and a twentieth for dry measures.
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.
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.
Mishnah eleven continues to deal with the weighing of produce.
Section one: Rabban 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 buyer’s side of the scale, which contained the goods, sink one handbreadth lower than the seller’s 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 one’s 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.

היה שוקל עין בעין – as for example, in a place where they did not have the practice of the customary additional weight in retail of a handbreadth and he restores the reed of the balance equally.

נותן לו את הגירומין – his customary additional weight in retailing for every ten liters, he adds one-tenth of a liter which is 1/100 when he sells liquid/moist, and with dry [products], he adds [one-half] of one tenth of a liter to every twenty liters.

למוד בדקה – with a small measure.

לא ימוד בגסה – with a large measure, for there is loss to the purchaser as he doesn’t give him anything other than the one customary additional weight in retailing. But he should need to give him many customary additional weights in retailing.

למחוק – to remove what goes upon the rim of the measure.

לא יגדוש – and even though he adds cost to him.

ימחוק – and even though he lessens for him from the payment.