You shall not have large and small weights in your pouch. (Deuteronomy 25:13)
It should go without saying that if a shopkeeper has weights and measures that cheat customers out of what they have rightfully purchased, they may not be used. What may be surprising is that we’re not even allowed to hang onto them. The reason for this is so that we should not be tempted into using them. If we get rid of them, we can’t be tempted into shorting others on their purchases.
This mitzvah applies to men and women in all times and places. Even if the weights and measures are never used, a person is in violation of this mitzvah merely for retaining the dishonest equipment. This mitzvah is discussed in the Talmud in tractate Baba Basra on pages 88a-90b. It is codified in the Shulchan Aruch in Choshen Mishpat 231. This mitzvah is #272 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #84 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.