Is it obligatory to calculate and pay out the bank interest that the father took from the inheritance, given the ignorance of its amount, knowing that he deemed it permissible based on a fatwa, and then withdrew his money from the bank three years before his death?

If the profits result from depositing money in usurious banks, then they are forbidden. However, what the father used from it due to his ignorance of its prohibition, there is no sin upon him, and his heirs are not required to give back its equivalent. As for what remains of these profits, it must be disposed of by spending it on Muslim welfare or giving it to the poor and needy. The fact that the money is an inheritance does not make it permissible to own it, but it is permissible for a poor heir to take from it according to his need. Unlawful money does not become lawful through inheritance. If the heir knows that the money is from usury, he must give it out and return it to its rightful owners if possible, otherwise, he should donate it as charity. If lawful and unlawful money are mixed and the amount is unknown, it should be divided into two halves. If the amount remaining from the profits is unknown, one must be cautious and give charity until one is certain that the unlawful portion has been removed.