My grandfather died and left some funds in the bank. His children wanted to purify his money from interest, and it was difficult for them to extract the interest from the original money, because he frequently made deposits and withdrawals. What should his children do? Should they estimate the interest and give it in charity? Will my grandfather be tormented, even if they give the estimated sum as charity, bearing in mind that he did not know that bank interest is Riba?
May Allaah Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The children of the deceased should give an amount, which they think is equivalent to the interest from the money left by their father, in charity. They need to know the original deposited amount; whether there were one or several deposits. Also, they should demand a bank statement distinguishing deposits from the interest.

On the other hand, it is hoped that Allaah, The Almighty, Will Forgive the deceased, Have mercy upon him, and Pardon what he had done out of ignorance.

What you do is useful to him, and it is a form of settling his debts and fulfilling the rights to which he is liable.

His children should frequently supplicate Allaah, The Almighty, to Have mercy upon their father and to Forgive him, for their supplication is very useful to him after his death. The Prophet, sallallaahu ‘alayhi wa sallam, was reported to have said: “When a person dies, all his deeds come to an end, except three: continuous charity, beneficial knowledge (which he has left behind), or a righteous child who will supplicate for him.” [Muslim]

Allaah Knows best.