What is the ruling on Zakat for money saved and sent to one's father monthly as a debt, and how is Zakat to be paid on the money accumulated monthly?

Previous fatwas have addressed the ruling and method for paying zakat on a monthly salary. As for the money the father took as a loan, zakat is obligatory on it every year if the father is wealthy and capable of repayment. However, if he is not, then zakat is not obligatory until the money is received, and it is then subject to zakat for one year.