Is buying from a person or dealing with institutions believed to deposit their money in interest-bearing accounts considered aiding in sin?

A person's engagement in usury—such as depositing money in a usurious bank—does not preclude dealing with him in permissible transactions, like buying a permissible commodity. Indeed, the Prophet, peace and blessings be upon him, used to engage in buying, selling, and borrowing with the Jews, even though they were known for taking usury and consuming ill-gotten gains. The Muslims after him also did the same.