Is money derived from the rent of houses built with usurious loans lawful or unlawful, and if unlawful, how can it be purified, and what is the ruling on benefiting from food, drink, and possessions in which lawful and unlawful money are mixed?

Working in a usurious bank is unlawful (haram), whether as a scribe for usurious contracts or a witness to them, due to the Prophet's (peace and blessings be upon him) hadith which curses the one who consumes usury, the one who pays it, its scribe, and its two witnesses, holding them equal in sin. Even if the work involves other tasks, it still constitutes assisting those involved in usury, and Allah has forbidden that. As for the father's earnings from the usurious institution, if he was ignorant of the religious ruling during his work and then repented, there is no blame upon him for the salary he received.