Is it forbidden to agree with a Christian friend that he gives me a monthly sum in exchange for my contributing financially to his business of mobile phone top-up cards?

Among the conditions for the validity of Mudarabah is that the capital is not guaranteed, and that a fixed profit is not stipulated. If these are stipulated, the contract becomes a usurious and invalid loan. In this case, both the capital and the profit belong to the owner of the capital, and the worker is entitled to a wage commensurate with his effort. All losses are borne by the owner of the capital, and the worker is entitled to a wage commensurate with his effort.