In the Tora it is written: (vayiqra 19:36) "Use honest scales and honest weights, an honest ephah and an honest hin ; I am the LORD your God, who brought you out from the land of Egypt." Epha and hin are Biblical units of measurements (like pounds or ounces). The Tora instructs the seller to keep his commercial scales well adjusted and clean. So there is no dishonesty in trade and business transactions. Otherwise, even a little dirt on the scale will affect the final weight. And the buyer will get less than what he or she is paying for.
Our Rabbis extended the scope of this Mitzva. To remember this rule they explained that the word hin --a liquid measure unit-- is similar to the word hen which in ancient Aramaic means YES. They said: in the same way we must have an honest scale (hin) we need say an honest "yes" (hen). In a business transaction our YES has to be always an honest YES.
Illustration: A client is willing to buy a car from me right now if I assure him that I will deliver the car at the end of the week. I know that I will not get the car by the end of the week. But I say to myself: 'I will close the deal now, and tomorrow or the next day I just tell him that there was an unexpected delay'. So I say YES and we close the deal. That would be an example of a dishonest YES. I should have told the truth to my customer. Even when I know that by saying NO he might go somewhere else to buy his car. If I lie to my customer and I say a dishonest YES I'm transgressing the Mitzvah of keeping a right YES, and a right NO.
Rabbi Eliezer Papo (1785-1828) in his book Pele Yo'etz assures that although in the short term the seller might lose a good deal, in the long term, the honest YES and the honest NO is not just the right thing to do but the smartest business decision as well. Because, he explains, there is no bigger asset for a seller than his good name and a good reputation.
by Mrs Lori Palatnik, from Aish.com