What is the ruling on breaking an oath of divorce in the event of helping a father purchase a car, if the father had bought the car without the son's knowledge or need for help?

There is no breaking of your oath regarding your vow to help your father purchase a car if your intention was not to spend the amount on the car's price. In that case, it can be spent on another aspect, for oaths are based on the intention of the one who swears them. And even if you intended the amount to be the price of the car, there is still no breaking of the oath if your father bought the car without your knowledge.