Is it a violation of Sharia for a father to cede his land to his son, who had bought him a car in installments, after the son had bought the car with the intention that its entire income would go to the father?

It is permissible to sell the car to your father in exchange for his land, provided that the father sells the land at market price without favoritism, and that you pay the difference between the values of the car and the land, if any. The father must be just among his children in gifting; therefore, he is not allowed to waive the price difference. If the sale and purchase are conducted correctly, the land must be documented in your name to preserve rights and prevent future disputes.