Is it obligatory to pay a monthly or annual sum to the poor and needy as expiation for dealing with banks and their interest?

If the parents deposited money in a usurious bank out of ignorance or by imitating someone who permitted it, they are not obligated to dispose of what they earned from it. And if the son does that on their behalf, out of caution and filial piety, there is no harm. However, if they knew the unlawfulness of it, they must dispose of the usurious earnings, and they must accurately determine its amount if they are unaware of it. The son may undertake that on their behalf after their knowledge and permission, while urging them to repent and seek forgiveness. It is not permissible to continue usurious transactions under the pretext of getting rid of the interest.