Is it permissible for a father to sell a plot of land and two daughters to his son to pay off his debts in installments, or for the son to pay off his father's debts on condition that the father repays him later, taking into account that the son is the only one working among his siblings?

If you wish to purchase the land from your father and pay off his debts in installments with his consent, that is permissible. However, it is better to pay off his debts and await an improvement in his financial situation so he can return your money; this is to prevent future disputes with your siblings. If you donate by paying off his debts out of filial piety and kindness to him, your reward will be greater, for the Prophet, peace and blessings be upon him, said: "The pleasure of the Lord is in the pleasure of the parent, and the displeasure of the Lord is in the displeasure of the parent."