Praise be to Allah the Lord of the Worlds.
In principle, Sharia forbids tampering with weighing scales and measuring instruments and commands fulfilling rights to the full. Allah says, "Woe to those that deal in fraud,-Those who, when they have to receive by measure from men, exact full measure,But when they have to give by measure or weight to men, give less than due."{Al-Mutaffifin, 1-3}.
But, if it is customary and agreed upon amongst dealers to weigh poultry according to a certain method then the jurisprudential maxim states that "Custom is an Arbitrator" and "What is known by convention is deemed as conditional." Usually, customary practices pertaining to some contracts are resorted to upon the emergence of problems or hardships resulting from adherence to the letter of the contract. These customary practices are put into effect to remove hardship or do justice to the contracting parties.
Poultry farmers follow a certain customary practicein calculating the weight of chicken. Such practice was established to protect the marketer from weight shortage resulting from the weight of the chicken box at the farm and the retailers shop, in addition to the kilo taken out from each box against chicken runoff. In this case, we rule that this customary practice is valid if it meets the following conditions:
First: It is a general customary practice amongst poultry farmers.
Second: Contracting parties are familiar with this practice and agreed on it.
Third: The weighing scale isn`t tampered with where the deducted weight is as agreed.
Fourth: The weight of boxes isn`t tampered with. For example, the weight of any box should be no less than (6kg) before the deduction, so if any box weighs (5.5) then the half kilo is ill-gotten property.
Fifth: No weight is deducted save what has been agreed upon.
In conclusion, there is nothing wrong with following this commercial practice if the above terms are met. And Allah the Almighty knows best.