What is the ruling of Islamic law regarding the refund of the price of a plot of land I purchased to build a house, when it became clear that part of it is owned by other parties, and the seller wishes to return the money at the old price, while I wish to recover what is equivalent to the current square meter price?

If a sale has been concluded, it cannot be annulled except by the option of istiḥqāq (claim of right), which occurs when the sold item is discovered to be owned by another person, either wholly or partially. In such a case, the buyer recovers the price he paid from the seller. It is mentioned in Al-Fatāwā al-Hindiyyah: "If the sold item or the usurped item is claimed as of right after it was sold or usurped, then its price is recovered." The buyer is only entitled to the price he paid to the seller. Scholars have defined istiḥqāq as: "A lifting of ownership by the establishment of another ownership that existed previously; for istiḥqāq, a boundary has been drawn."