Is it permissible for the buyer to sell the land to another party for a higher price before settling the full amount and transferring ownership, and is it permissible for the seller to cancel the contract and reclaim the land in exchange for returning the amount paid?

If the land has been sold and the buyer has taken possession of it, he is permitted to sell it for any price he wishes, as it has become his property. The registration is for documentation only. The fact that he has not paid the remaining price does not prevent him from disposing of it, unless it was stipulated that he should not sell it until the full payment is made. This condition is permissible. If there was no such condition, he cannot be prevented from selling it. The seller is not allowed to rescind the sale due to envy, unless he was deceived in the price (option of gross deception), in which case he has the choice between rescinding the contract or upholding it. If there was no option of gross deception or a prior condition, the sale is binding and cannot be rescinded except with the mutual consent of both parties.