Is it permissible to use the saved amount to buy a simple used car to facilitate movement for the wife and son, which would delay debt repayment, knowing that using bicycles as a means of transport causes great hardship for the wife and son, especially in winter and rain?

You must pay your debt to your father immediately if he asks for it, and you are not allowed to postpone the payment to buy a car. However, if your father does not demand immediate payment, there is no sin upon you for delaying it. The evidence for this is that a mature debt owed by one who is able to pay must be fulfilled immediately upon the creditor's demand, as the Prophet, peace and blessings be upon him, said: "Procrastination by a rich person is oppression." If the creditor does not demand it, then it is not immediately obligatory.