What is the legitimate Islamic formula for financial dealings in currency trading that combines capital management with a percentage of profit (20%) without bearing losses, and some investors’ demand for a fixed monthly income for two years with the return of the original capital at the end, knowing that the latter scenario is impermissible? Is it permissible for an investor to buy a car and sell it to the manager in installments so that its price can be invested?

It is forbidden to guarantee the capital in a Mudarabah (partnership where one provides capital and the other labor) due to the risk of it leading to usury (riba). Specifying a duration for the Mudarabah is permissible if both parties agree and no harm befalls the worker. The capital owner is not entitled to any profit except through the Mudarabah, which exposes him to the risk of loss. As for buying a car in installments from your friend after he has bought it for himself and it has entered into his possession, without you being obligated to buy it from him, there is no harm in it. This is similar to the issue of Tawarruq (monetization).