Is it permissible to benefit from and dispose of a large fortune of unknown origin, left by the deceased father in a bank abroad? And if not, what should we do with it?

Your father's money—if he was an upright person and his work was permissible in the country—is lawful, and you are permitted to divide it. As for usurious profits, they must be disposed of by spending them on general charitable causes. It is not permissible to benefit from them because they are ill-gotten gains. You are not required to scrutinize his wealth as long as he is known for his integrity. However, if you know that his capital is unlawful, then it must be disposed of by spending it on charitable causes.