Is my action of not praying behind an Imam who issues fatwas permitting dealings with usurious banks, and who has other fatwas such as eating sweets during prayer not invalidating it, correct, and is it permissible to pray behind such an Imam?

What usurious banks do, lending and borrowing with interest, is precisely the forbidden Riba. And the person who issues a fatwa (definition) permitting this should fear Allah and know that he bears the burden of sin for everyone who acts upon his fatwa, because the Prophet, peace and blessings be be upon him, said: "Whoever is given a fatwa without knowledge, its sin is upon the one who issued it." It is permissible to abandon praying behind this Imam if he will pray behind someone better than him, otherwise, he should not abandon the congregational prayer.