337. Caveat Emptor: The prohibition against overcharging or underpaying

There are fair market values for things being sold. If the seller overcharges or the buyer underpays by more than one-sixth, the sale can be voided. (That particular law only applies with movable property but we’re not allowed to cheat one another in real estate transactions, either.) You will notice that the verse specifically prohibits not only the seller from cheating the buyer but also the buyer from cheating the seller.

Only the one who was cheated can void the sale. If a person overpaid for an object and the seller for some reason wants to undo the transaction, the buyer is under no obligation to do so. (See Talmud Baba Basra 84a.) An unbalanced sale is not eternally reversible; the time limit to complain is as long as it would take the buyer to bring the purchase to be evaluated (Baba Metzia 52b); this time limit does not apply to the seller, as the object is no longer in his possession.

The reason for this mitzvah is simple enough: cheating one another is wrong. Not only that, it shows a lack of faith in God. We’re not going to benefit by lying, cheating and breaking His laws. The best way to be prosperous is to deal fairly, for which He will bless us.

This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the fourth chapter of tractate Baba Metzia, on pages 49b-59a and is codified in the Shulchan Aruch in Choshen Mishpat 227. It is #250 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #47 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.