What should one do who took out a loan, the impermissibility of which became clear to him three years later, and he is unable to stop the payments? Can he purify the money after the loan ends, and is all the money earned from the project unlawful?

The borrower must repent to Allah for the usurious loan and repay the loan and its usurious interest by selling the project, provided that this does not entail an unbearable hardship. If he is unable to do so, he is not obliged to sell it, and there is no harm in benefiting from its income, because the loan, once seized, enters into the possession of the borrower and becomes a debt upon him, and he is obligated to repent from dealing with usury.