What is the ruling on visiting a sister who is married to a man who works in a usurious bank, and eating and drinking from his money? What is the ruling on accepting gifts from him? And what is the ruling on breastfeeding her child with her milk, or with formula milk bought by the husband with his money?

Working in usurious banks is not permissible. Some scholars are of the view that money earned through unlawful means—such as usury—does not prohibit interacting with the one who earned it, because the prohibition is attached to the earner's responsibility, not to the money itself. They supported this by the fact that the Prophet, peace and blessings be upon him, used to interact with the Jews, even though they consumed unlawful earnings and took usury. Therefore, there is no sin upon the one who benefits from the money of an employee in a usurious bank, but he should advise his sister about the impermissibility of working in the bank.