What is the ruling on previous profits that were received from an investment deposit in a bank that was later found to have usurious transgressions in its dealings, knowing that the bank had previously stated that its dealings were Islamic?

The permissibility of dealing with Islamic banks is conditional upon their adherence to Sharia regulations. If it becomes clear that a mixed bank's transactions include both permissible and impermissible elements, then the impermissible portion of the profits must be disposed of by spending it on the welfare of Muslims or on the poor and needy. This should be done by estimating the amount as precisely as possible; otherwise, by exercising ijtihad (independent reasoning) and preponderance of opinion. Ignorance of violations negates sin for the transaction, but it does not permit benefiting from the impermissible profits. However, if one is not certain that the bank deals in usury or other forbidden transactions, then there is no obligation to dispose of any of the profits.