I learned in the Gemara (BB 88B) that when someone sells by weight he must give the customer more than the exact weight that he is charging for, and in fact the Gemara derives this rule from a pasuk. It seems that this law is violated today when one buys packaged goods where the price is marked for the exact weight of the package or when the store weighs out the quantity being sold on an electronic scale and the price one pays is for the exact amount that the scale shows. In both cases it seems that this law is routinely violated. Is there a basis for our custom not to fulfill this obligation?
Before addressing your question let us review the Gemara. The Gemara says that there are two ways in which one can fulfill this law.
In the time of the Gemara weighing was done on a balance scale. A scale had two trays. On one tray one placed the item he wished to weigh and on the second the seller placed objects of known weight (called weights). The selling weight was determined by balancing the purchased item against the weights and the total weight that was needed in order to maintain the scale at the equilibrium was the weight of the purchased good.
The Gemara says that the seller is required to add to the tray containing the item that is being sold an amount that will cause it to be not at equilibrium but rather a tefach lower than the tray containing the weights. The Gemara says that a community may adopt an alternative custom in order to fulfill this law. Sellers may determine the precise weight of the good being sold and charge that amount, but then the seller must add a fixed amount. The Gemara says the amount that is added must be at least one percent when selling liquids and a quarter of a percent when selling solids. The Rashbam explains that reason for this difference is that when one measures liquids the customer always loses a little bit which remains in the receptacle.
The Gemara derives these rules from the pasuk (Devorim 25, 15) that commands us to have honest weights and measures. The Gemara derives from the words used by the Torah that the pasuk not only commands us to have honest weights and measures but that the seller (who is the one who maintains the weights and measures and is being commanded to have honest weights) must give a little more. The statement of the Gemara is that the seller must give “from what belongs to him.” From this we see that the prohibition is not against a kind of stealing, but rather an additional command on a seller to give more than the customer deserves.
Since this command is not part of the prohibition against theft, the Minchas Chinuch (mitzva 259) and Maharam Shick (CM 30) derive that the law only applies if the customer is Jewish since we are only forbidden to steal from or to sell to a gentile using dishonest weights and measures. (The Rambam (Geneivo 7, 8) explicitly states that the latter is in addition to the usual prohibition to steal from a gentile.) This additional command to add does not apply if the customer is not Jewish. Similarly, a gentile, who is forbidden by the Torah to steal, is not commanded to give extra since it is an additional amount and one who fails to give it is not a thief.
The Minchas Chinuch points out that if a seller uses dishonest weights he will certainly also violate the command to give extra.
Note that Tosafos explicitly writes that the command to give extra is a Torah command. Even though sometimes the Gemara writes a Torah source for a rabbinic edict, here this is not the case. Therefore, your question is a Torah issue.
Your question was posed to the Maharam Shick (CM 30), a prominent disciple of the Chasam Sofer. Even though they didn’t have electronic scales, nevertheless, sellers in his time did not abide by this law. He justifies this practice after-the-fact.
In order to arrive at a leniency he first proves that the Torah’s rationale was that Hashem wished to ensure that the customer will not be shortchanged. He proves this from the fact that when one sells a piece of land there is no requirement to add anything. Even though the same pasuk that is the source that one must add when selling by weight also discusses measurements that include measurements of land, yet, when it comes to weighing, the Gemoro derives that the seller must add but when he sells fixed property he does not need to. He explains that the difference is that when one measures a parcel of land there is no danger that the customer will be cheated, whereas when one weighs there are many. From this he derives that the Torah wrote this law in order to ensure that customers will not be shortchanged. It should be noted that R. Gershom in his commentary implies this as well.
He then discusses whether a buyer may waive his right to receive extra. Based on two rulings of Rishonim he arrives at the conclusion that, while an individual may not waive his right to receive extra, but if everyone waives their right, it is allowed.
One of his sources is a ruling of the Rashbo (res. 2, 292, cited by Beis Yosef CM 417) concerning digging under the public thoroughfare. Whereas the Mishna (BB 60A) rules that one is not allowed to dig under public property, in the time of the Rashbo it was common for people to dig under public property in order to drain rain and sewage water from their property. The Rashbo justified their practice based on the fact that everyone in all places dug under the public thoroughfare. Therefore, he says, since it is a general necessity, there is a tacit agreement of all to allow each other to violate the ruling of the Gemara.
The Maharam Shick analogizes to the case of common sales. Since everyone violates the ruling of the Gemara, we view the situation as if there is a tacit agreement among all to waive the Torah law.
It should be noted that the derivation is quite weak. One difference is that in the case of the Rashbo there was a need to relax the ruling of the Gemara, a need that did not exist at the time of the Gemara. However, in our situation there is no need to relax the law. Additionally, the rule forbidding digging under the public thoroughfare is only a rabbinic ordinance in order to maintain public order. Since that was the entire purpose, if it is the public interest to change the rule, it can be changed. However, in our situation the law to give extra is from the Torah. We do not have a proof from there that the public may agree to remove a practice that was instituted by Hashem in the Torah.
The Yerushalmi (BM 7, 1) says that custom changes the law and one might want to apply that principle in our case. However according to many, the principle of the Yerushalmi that custom changes the law, applies only to two situations. One situation where it applies is to laws that were made in the absence of custom, for example the hours that an employee must work. Even though the Gemara gives a rule, if the custom differs we abide by the custom since the Gemoro’s rule was only made as a default, in the absence of a custom. However, the law that one must give extra was not just given in the absence of a contravening custom. Therefore, this does not justify the Maharam Shick’s ruling
The second situation where the Yerushalmi’s rule of custom changing law applies is if the custom is sanctioned. The consensus of the Rishonim (Ramban (BB2A), Ohr Zorua (BM 282) and many others) is that only if the Rabbis agree to waive the law or, if there is a vote by the elders of the town institute a custom, is the law waived by the custom.
Therefore, the leniency of the Maharam Shick is somewhat strained. He himself notes this and he concludes that a G-d-fearing person should not rely on any leniency and should give extra.
The sefer Mischar Kehalocho suggests that, based on the principle of the Maharam Shick and Rabbeinu Gershom that the basis for the law is to ensure that customers should not be shortchanged, there is room for leniency with electronic scales since they measure almost exactly and just round-off by a few grams which sometimes works to the benefit of the customer. We note that the Birchas Avrohom in his commentary writes that storekeepers routinely violate the law when they use electronic scales.
It is possible to use an electronic scale and to fulfill the law by setting the scale at minus a few grams. We note that the Pischei Choshen requires setting the scale at a negative amount since the scale rounds off the amount being weighed. Therefore, he says that one must set the scale at a setting that the customer will never be charged more than the amount he is buying. Alternatively, one can avoid this problem by informing customers that the scale rounds off.
The Chashukei Chemed remains with a doubt whether a storekeeper who sells prepackaged goods which are sold by weight (e.g. a kilo of sugar) and where the packager failed to add, violates this law. We suggest that perhaps the prohibition does not apply since the storekeeper does not really sell by weight. He really is selling bags which are called “one kilo bags” but he does not really sell by weight but by the number of bags.
In conclusion: For generations many people have been lax with the Torah law that the seller should add to the weight. One who is careful to adhere to mitzvos should be scrupulous with this law as well and some stores do adjust their electronic scales. However, there is a basis to justify one who is not scrupulous.