What should a father do who has been slandered and defamed by his brother, after he financially helped him, and the brother is now seeking to seize his property?

A father helping his uncle is an act of maintaining kinship ties and benevolence. Allah helps His servant as long as the servant helps his brother. The uncle's ill conduct does not justify boycotting him. He has a right based on kinship, and maintaining ties with him carries a great reward. "If you are as you say, then you are feeding them hot ashes, and you will continue to have a supporter from Allah against them as long as you remain upon that."

As for the money: If it was a donation or a gift: The father has no right to demand its return, unless he was the donor's father. If it was a loan: The uncle must repay it, unless he is in hardship, in which case he should be given time. If it was on the basis of partnership: The uncle is a partner according to the agreement.

It is advisable that wise people mediate between them to reconcile their differences and foster mutual satisfaction and forgiveness.