What is the ruling on financial dealings with a company that provides a fixed monthly return (1.5%) in exchange for depositing a sum of money? Is this transaction considered usury (riba)? If so, is it permissible to spend from this money in the absence of any other source of livelihood, and what should the beneficiary do with this return if it is usurious? What is the ruling if the company refuses to return the money and offers an unspecified profit percentage?

Your agreement with your friend that he pays you a fixed percentage of money in exchange for you depositing a sum of money with him is explicit usury (riba). The Sharia (definition)-compliant solution is Mudarabah (definition), where you become a partner in profit and loss, and the percentage of profit is variable. What you called a "tender," if its percentage is fixed, then its ruling is the same as the previous contract (usury). But if it's a percentage of variable profits, then it is a legitimate Mudarabah. It is not permissible to continue with a usurious contract, as it is consuming prohibited usury, and the absence of another source of income does not make it permissible. Rather, it is possible to borrow to cover essential needs. To get rid of usurious gains, they must be spent on charitable causes, such as for the poor and needy. If your friend offers you an unspecified percentage of the profit (and not a percentage of the capital), then this is what is required to rectify the transaction.