Is a father considered sinful for selling a plot of land to his son in exchange for prior assistance and a sum of money? What is the Sharia ruling regarding the son's ownership of this land after the father's death, especially since the promise was made only with a handwritten document? Is it permissible for the son to relinquish it to his siblings, or is it a right for his children?

The sale that took place between you and your father concerning the plot of land is valid, as its pillars and conditions were met. Since you paid its price, the plot is rightfully yours, and there is no blame upon you for taking it, nor anything upon your father. However, if the heirs doubt the sale or are not pleased with it, you must prove it with Sharia-compliant evidence. It is advisable to reach an agreement with your siblings to avoid animosity.