Is it permissible for me to purchase a piece of land that I have been authorized to sell, at a price lower than that agreed upon with the buyer, without informing the principal or the original buyer about this?

It is permissible to buy land from a relative, either for cash or on credit, provided that the relative knows its value and is skilled in buying and selling, and then to sell it to the one who promised to buy it or to someone else. However, if the owner is ignorant of the land's value and is not good at bargaining (i.e., is easily deceived), then a sale with an exorbitant price gives the owner the option to annul the sale. It is disliked to engage in such a sale if it involves defrauding someone who is easily deceived, and one must advise them if they seek advice, because defrauding them in such a case constitutes prohibited deception. The hadith: "Defrauding one who is easily deceived is usury" is weak.