Is it considered deception for a merchant to ask the supplying company to reduce the weight of a 50 kg bag of corn, stated as such, to 48 kg, so that the selling price aligns with his profits and shop expenses, while other merchants do so by reducing the weight to 45 kg?

What your father is doing by reducing the weight of the sack from 50 kg to 48 kg is considered deception and short-weighing, and it is forbidden by Islamic law, based on what was authentically reported from the Prophet, peace and blessings be upon him: "The seller and the buyer have the option as long as they have not parted. If they are truthful and clarify, they will be blessed in their transaction. If they lie and conceal, the blessing of their transaction will be wiped out."

This also falls under the warning mentioned in the Almighty's saying: "Woe to those who give less [than due], those who, when they take a measure from people, take in full but if they give by measure or by weight to them, they cause loss."

The duty is to advise and be truthful in trade, and to avoid deception and short-weighing. The actions of other merchants do not justify this act.