What is the ruling on my working with a Muslim who repairs garage doors and replaces old, functional parts with new ones to earn money, knowing that I have advised him repeatedly and he prays and fasts? Is this considered earning unlawful money?

If the employer deceives customers by claiming that an old part is damaged when it is still functional, in order to replace it with a new one, this is deception and fraud. The Prophet, peace and blessings be upon him, said: "Deceit, fraud, and treachery are in the Hellfire," and "Whoever cheats us is not of us; and deceit and fraud are in the Hellfire." Therefore, it is not permissible to work with him in matters that involve deceiving people, and the earnings from it are ill-gotten. The fact that the customers are disbelievers does not permit deceiving them.