What is required of me to clear my conscience of a sum of money I received by mistake, knowing that I cannot return it now, and if I am able to, to whom should I return it?

Since you have erred by taking money to which you have no right, you must repent to Allah by regretting your action and resolving never to return to that sin. You must also hasten to return the money to its owners, even through indirect means. If it is impossible to deliver it to its rightful owners, then give it as charity to the poor and needy.