In the Tora it is written: (vayiqra 19:36) "You shall have just balances, just weights, a just epha, and a just hin; I am the LORD your God, who brought you out from the land of Egypt." Ephaand hin are Biblical units of measurements (like pounds or ounces today). The Tora instructs the seller to keep the scales balanced (=just) and well adjusted, so there is no dishonesty in trade and business transactions. If you sell tomatoes you need to maintain the scale clean and well adjusted. Otherwise, even that little dirt on the scale will affect the final weight, and the buyer will get less tomatoes than what he paid for.
Our rabbi extended the scope of this pasuq and explained that the word 'hin' a liquid measure unit, is similar to 'hen' which in ancient Aramaic means YES. They explained that in the same way our scales need to be just, our words need to be just. When the seller is going to say YES, it has to be a just and honest YES.
Illustration: If I sell cars and a client says that he wants the car at the end of this month, and I know that I will not get the car at the end of the month, I must tell the truth to my customer. Even when I know that by saying NO he will go somewhere else to buy his car, and I will lose the sell. If I lie to my customer and I pronounce the dishonest YES, I will be transgressing the Mitzva of HIN TZEDEQ: Keeping a right YES, and a right NO.
Rabbi Eliezer Pappo in his book Pele Yo'etz assures us that although in the short term the seller might lose a good deal, in the long term, the right YES and the right NO is the smartest business decision. Because, he explains, there is no better advertisement for a seller than the fame of a good name and honesty.