What is the ruling of the Sharia on a transaction that involves investing a sum of money to purchase a car and lease it for a fixed monthly amount, then selling the car while continuing to pay the agreed-upon amount, and in the event that the invested amount later decreases under the pretext of "car depreciation"?

The questioner seems confused in explaining her situation regarding the money she paid to her cousin. Her case can be summarized into two possibilities:

The first possibility: She appointed him as an agent to purchase a car and lease it to her for a fixed monthly sum, with any excess profit belonging to him. This is permissible, and the agent must hand over the car's rent to his principal. Upon termination of the contract, he must return the car in its original condition or pay its full price if he was authorized to sell it. If he sold it without authorization, he is liable and must immediately hand over its value or price.

The second possibility: She paid the money to invest it in his company. This is forbidden if she stipulated a specific monthly amount; rather, the profit must be a common percentage. Therefore, this transaction is forbidden and must be annulled, and the capital must be returned in full if no loss occurred. Her cousin is entitled to a fair wage for his work, and his claim of deducting 15,0 for car depreciation is baseless and an unlawful appropriation of money.