What is the solution for someone who took money for investment with a fixed profit and then learned of its prohibition, and now cannot return it for fear of harming his family and being imprisoned?

The transaction mentioned is "Mudarabah" (profit-sharing partnership). Its correct form is that the profit is divided according to the agreement, and the loss is borne by the capital. However, stipulating a fixed amount or a fixed percentage of the capital as profit is unanimously prohibited. The money must be returned to its owners immediately, as procrastination by the rich is an injustice, and delay makes his honor and punishment permissible. The debtor must pay what he can from people's money, and the remainder will be a debt upon him until he is able to pay it.