Is it permissible for two friends to agree that one of them will repair the other's car, provided that the first one handles all matters related to the repair in exchange for reimbursement of the paid costs and a 20% profit margin, knowing that the actual cost cannot be precisely determined until after the work is completed?

If the cost is known and specified, then it is permissible for the worker's wage to be a percentage (e.g., 20% or 10%). However, if the cost is not specified and the wage is a percentage of it, then this is not permissible due to the unknown nature of the wage, as the Prophet, peace and blessings be upon him, forbade hiring a worker until his wage is made clear to him. This is the view of the majority of scholars. Some scholars, however, have permitted it, such as when someone gives yarn to be woven for a third or a quarter of its price, or if he says, "Sell it for such-and-such, and whatever profit there is, it is yours." The solution is to specify a fixed wage for the worker or for him to specify it himself, and then authorize him to purchase spare parts and complete the work.