What is the ruling on bank interest and similar matters, and what is the ruling on someone who takes it following fatwas that permitted it, if it is in fact unlawful?

Usurious interest is forbidden, whether from a bank or any other source. The opinion that it is permissible is anomalous and invalid. Whoever took usurious interest based on an erroneous fatwa must dispose of it by spending it on charitable causes if it still exists. If he has already spent it, then there is nothing upon him, and the sin is on the one who issued the fatwa to him, based on the Prophet, peace and blessings be upon him, who said: "Whoever is given a fatwa without knowledge, his sin is upon the one who issued the fatwa." However, if the Sharia ruling becomes clear to the questioner and he turns away from it, then his sin is upon him.