What is the ruling on marrying a young man who has a lawful job, while his parents work in a usurious bank? And what is the ruling on using his parents' money to buy the marital home and its furnishings, to spend on his wife from it, and to eat food in his parents' home? Should the proposal be accepted or rejected?

If the suitor is righteous, then he should be accepted. As for the money, if your parents have lawful money in addition to the money earned from riba-based banks, then accepting a gift from mixed money is permissible, though disliked. However, if all their money is earned from working in riba-based banks, then there is a difference of opinion among scholars regarding the permissibility of a child benefiting from their wealth. Some scholars have held that it is permissible to accept maintenance and gifts from someone whose wealth is unlawfully earned due to its acquisition, not its essence. This is the view given in a fatwa by Sheikh Ibn Uthaymeen, who reasoned that the Prophet, peace and blessings be upon him, used to accept from the Jews and eat from their food, even though he knew they dealt in usury. Therefore, the money is lawful for you and not unlawful.