All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If a person pays an amount of money to another person in order to invest it in trade or industry and the like in return for a known percentage of its profit, then this is considered a Mudhaarabah (profit sharing).
However, there are some conditions for the Mudhaarabah to be valid, among which are the following:
A- The person holding the capital money should not guarantee the money unless he is negligent or abusive.
B- None of the two parties should condition for himself a fixed amount of profit; rather, a known percentage, like one-half, one-third, or the like. For more benefit, please refer to Fatwa 94668.
Therefore, the agreement that took place between you and your cousin is a non-Sharee’ah compliant agreement; so on what basis does he give you half of the profit of the garage? However, if he started business with your capital money and there was a profit, then you deserve it according to the agreed upon contract.
On the other hand, since the capital money is still available in cash and your cousin did not invest it, then you have no right to anything of the profit of his garage. Therefore, you have to conduct a valid contract if you wish to continue dealing with each other, because the first contract is void as it is not clear whether it is about the new garage or the existing garage.
Allaah Knows best.