Is it considered cheating to change the length of trousers from 108 cm to 105 cm to compensate for profit after merchants forced my father to lower the price, knowing that the merchants see and approve the sample? What is the ruling on the money earned from this sale, and how does one repent if it is considered cheating?

If what you mentioned involves deceiving buyers by concealing the true description of the commodity, such that if they knew it, they would not buy it at that price, then it is forbidden deception (ghish). Otherwise, it is not considered deception.

Deception is forbidden by Islamic law and destroys trust. It includes all types of deceit, such as: lying about the price, misleading others into believing there is a desire to buy, enticing with false means, concealing hidden defects, and excessive overcharging (ghabn fahish). The pressures faced by merchants do not permit deception.

Repentance from deception requires remorse, cessation of the act, and a firm resolve not to return to it. In addition, the seller must seek absolution from those he deceived by returning the amount by which he deceived them, or by explicitly seeking their forgiveness.

Deception differs from usury (riba). Money taken through deception is taken without the owner's consent, and ignorance of its prohibition does not waive the right of the deceived. Not all the money becomes unlawful, but only the amount representing the price of the deception. If the amount is unknown, one should strive to estimate it in a way that most likely clears one's conscience.