It is Fardh (obligatory) when trading that a person weighs and measures goods fairly. One must never give the customer less weight or measure because severe warnings have been sounded in this regard. Allaah says, “Destruction be to the cheaters who, when they take measure from mankind (when they are buying from others), demand (that their purchase be given to them) in full. (However,) If they measure or weigh for people (when they are the sellers), they reduce (the weight or measurement of the commodity so that the buyer gets less than he pays for)”.
[Surah Mutaffifeen, verses 1-3]