What is the ruling on the son and his wife dealing with the unjust father of the husband, who denied receiving a sum of money that the son and his wife had fully paid, and is demanding it again?

The wife has done well by helping her husband pay off his father's debt and by patiently enduring the father's ill-treatment. It is from the etiquette of Islam to respond to evil with good. If the repayment of the debt is proven, the debtor's liability is discharged, and he cannot be demanded to pay again. The issue of proving the debt is a judicial matter, not a fatwa, and requires evidence and arguments. If the father denies the debt and demands it before the judiciary, the spouses must present their evidence, and the judge will rule on that. The husband's discharge from the debt does not negate the obligation to honor his father, even if the father is unjust, for Allah Almighty says: "But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with appropriate kindness."