Is it permissible to consider the amount owed to the father as the cost of furnishing the son's house to waive the zakat for three years? What is the ruling if the son later reclaims the amount and buys a ready-made house instead of furnishing his own? Is it permissible to pay zakat money to the father to settle a debt? Can the son keep a portion of the debt paid with zakat if the father later repays the full amount, knowing that the son's house is still under construction and that he contributes more than 50% of the father's household expenses?

The debt owed to your father, which is in your trust, is subject to zakat (definition). The more cautious and absolving approach for one's conscience is that its zakat becomes due upon its retrieval for all past years. It is not permissible to count the debt you owe your father as part of your zakat. Likewise, it is not permissible to give your father zakat on the condition that he uses it to repay your debt. However, if your father is poor, and you give him zakat unconditionally, and he then uses this money to repay your debt, that is permissible for you. As for your father's maintenance, if he is unable to provide for himself, his maintenance is obligatory upon you and is not counted as zakat, unless you yourself are unable to maintain him.