Is dealing with the "Tahweesh" system offered by a company for installment sales of goods and electronic payment services, where customers deposit money with the intention of saving it for a variable return and without administrative fees, considered usury (riba)?

The consideration is not the change in profit, but rather the reason for the profit, its manner, and its condition. If the saving system is based on a legitimate Mudarabah (definition) (profit-sharing) contract and the investment of the money is permissible, then it is allowed. However, if any condition of Mudarabah is violated, such as the company guaranteeing the capital, then it is not valid, regardless of whether the profit is known or variable.