What is the ruling on a deposit placed in an Islamic bank that was later found to have mixed dealings, both Sharia-compliant and usurious, and must all of the returns be disposed of, or only the proportion from the usurious dealings?

The principle concerning Islamic banks is that they operate in accordance with Sharia, observe Islamic regulations in their transactions, and invest depositors' money through permissible means. Therefore, there is no objection to depositing money in them for investment and benefiting from the profits. If you have any doubt about a specific transaction, refer to the bank's Sharia Supervisory Board. If you know that a portion of the profits results from prohibited transactions, then only the prohibited portion must be disposed of, and the remainder of the permissible profits is lawful for you. Ibn Taymiyyah said: "That which is acquired through prohibited contracts, such as usury and gambling, if it becomes mixed and indistinguishable from other funds, does not render all of it prohibited. Rather, the amount of the former is distinguished from the amount of the latter. Then, this portion is given to its rightful owner, and that portion to its rightful owner."