Is money inherited from a maternal aunt lawful or unlawful, given that she used to deposit her money in both interest-based and Islamic banks, and the proportion of each cannot be definitively determined? This money was spent on renovating the house, so does this entail an expiation? And is the money that the maternal uncle returned to my mother, which is part of this inheritance, lawful or unlawful? If it is unlawful, must I donate it or return it, and what is the ruling on using the remainder of it to purchase something?

Unlawful wealth is not owned, even through inheritance. Rather, it must be disposed of by spending it on the welfare of Muslims or by giving it to the poor and needy. If it mixes with lawful wealth, the unlawful portion must be extracted. If the unlawful wealth is from usury (riba), it must be returned to the usurer, or given as charity on his behalf if he cannot be found. If it is from injustice, the same is done with the wronged party. If the amount of unlawful wealth is unknown, an effort should be made to estimate it so that the lawful wealth remains pure. It is permissible for the recipient of unlawful wealth, if he is poor, to give it as charity to himself and his family.

Accordingly, the questioner's mother must determine the amount of usurious interest and disburse it for charitable purposes. If she does not know the amount, she must err on the side of caution. As for residing in the house and benefiting from it, there is no harm in that. What was given to the uncle, or paid off as debt, or given as a gift, is not to be considered. For the prohibition relates to the liability (dhimmah) and not to the specific asset itself.