What is the ruling on a father spending from his money on his son's marriage, given that he is indebted with usurious loans to banks, which he took out of necessity to save his business from bankruptcy? Does the ruling differ between the necessities of marriage and its luxuries? And what is the ruling if his sincerity in not returning to usurious transactions without necessity is not proven?

Borrowing with interest is forbidden usury, unless it is for an urgent necessity. If this is established, then know that once a loan is taken, it becomes the property of the borrower and a debt upon him, whether it involves usury or not. It is permissible to accept what your father spent on your marriage, whether it was for necessities or luxuries, because after he took possession of these loans, they entered his ownership and became a debt upon him. If your father deals with usury without necessity, then you must advise him and explain the seriousness of that.