Is it permissible for a father to waive all or part of a debt owed by his adult son, and is this considered a prohibited gift that necessitates equality among siblings, especially if the father fully covered the marriage expenses of one of his sons due to his poverty, and then that son became wealthy, while the father only partially assisted his other children?

If a father lends money to one of his children to buy a house, he is not permitted to exempt them from repaying the loan unless all their siblings agree, or unless he gives them an equivalent amount. This is due to the obligation of justice and fairness among children in giving gifts. However, a father's assistance to his children in marriage falls under the category of maintenance (nafaqa), in which case he gives to each according to their need.