Is receiving a fixed monthly sum in exchange for a percentage of the profits from a supermarket whose monthly earnings are difficult to determine considered usury (riba), and what is the solution in this case?

This partnership is a Mudarabah (profit-sharing agreement), and its invalidity lies in the guarantee of the capital and the determination of a fixed profit. A Shari'ah-compliant Mudarabah is only valid if there is an agreement on a shared, known percentage for profit distribution. The difficulty of monthly liquidation can be addressed by treating the fixed amount as an advance payment, then conducting an inventory and liquidation periodically to determine the profit and settle accounts between what is due to him and what he has borrowed. If there is a loss, it is borne by the capital.