Is the full amount resulting from the sale of the car the property of the questioner, or does his father have a share in it, and what is required of him in this situation?

If the father stipulated that his son pay all his money to buy the car, and the son hid ten thousand Riyals from him, which the father then paid, then the son must return it to the father with repentance, unless the father forgives him, for the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions in what is lawful." Direct repayment is not mandatory. As for the remaining amount (146,0 Riyals), it belongs to the son. However, if he owns nothing else, he must return ten thousand from it to the father unless the father forgives him.