What is the ruling on taking the additional $500 paid by the seller, after receiving a refund of $3,000 as the price for the car whose value has increased?

When a sale fulfills its conditions and pillars, it becomes a binding contract for both parties. Thus, the buyer is obligated to pay the price and the seller is obligated to deliver the sold item. Neither party has the right to unilaterally annul it except with the consent of the other party, based on Allah's saying: "unless it be a trade conducted by mutual consent." If the seller wishes to withdraw and the buyer agrees to it in exchange for an additional sum, there is no objection.