Is it permissible to make a fictitious agreement with someone to purchase a plot of land—which is under dispute between me and my siblings—and then for me to buy it from him later, so that its full ownership devolves to me?

It is not permissible to use stratagem to acquire inheritance, but if the father wishes to sell his share, the son has a greater right to purchase it. If he sells it to someone else, the son has the right of pre-emption. The same applies if the share devolves to the heirs and they wish to sell it. It is not courteous to discuss inheritance with one's father.