Is money earned from working in a workshop that repairs functional engines by labeling them as defective considered unlawful gain?

If the workshop owner tells his customer that the motor needs rewinding, while the reality is contrary to that, then this is lying, cheating, and unlawfully consuming people's wealth. It is among the forbidden acts that necessitate repentance and returning the excess money to its owner.

As for the position of the questioner (the worker), he must advise the workshop owner and explain the impermissibility of his action and the obligation of repentance. If possible, he should inform the motor's owner of the truth, and this is from the perspective of aiding the oppressed.