Is replacing a spare part in a rented car without the owner's knowledge, while giving the owner the difference in cost, considered an unlawful act? And what should be done if it is?

What you and the tenant did is an unauthorized disposition of someone else's property, which is an aggression and a betrayal. This is based on the Prophet's (peace be upon him) saying: "All of a Muslim is inviolable to another Muslim: his blood, his wealth, and his honor." Therefore, you must return the car's spare parts to its owner, and it is not permissible for you to possess them.