Is it permissible to deduct 500 Saudi Riyals from a car sale down payment if the buyer refused to complete the transaction after paying a portion of the amount?

It appears that the sale transaction has been completed, fulfilling its pillars and conditions. Therefore, it is not permissible to annul the contract after its completion. However, it is recommended for one party to the contract to respond to the other if they request its annulment (iqālah), due to the Prophet's (peace be upon him) saying: "Whoever annuls a transaction for a Muslim, Allah will annul his stumbles." The one who annuls the transaction should not take any compensation from the one for whom it was annulled. Rather, the buyer retrieves the price, and the seller retrieves his merchandise. This is based on the view that iqālah is an annulment, which is the more prevalent opinion.