Dvar Torah by Rabbi Zelig Pliskin 

The Torah states: “You shall do no wrong in judgment, in measure of length, in weight, or in measures of content. Just scales, just weights and a just Ephah (a dry measure) and a just Hin (a liquid measure) shall you have” (Lev. 19:35-36).

The Torah forbids us to deceive both Jews and non-Jews when measuring or weighing (Choshen Mishpot 231:1; Chinuch 258). Furthermore, it is the fulfillment of a positive commandment to weigh and measure accurately (Chinuch 259).

The Chofetz Chaim wrote that the Torah enumerated scales, weights, ephah and hin to stress the severity of this prohibition; every type of deception being explicitly prohibited. A person must be very careful to ensure that whenever he sells something, he gives the customer the correct amount.