Am I committing a sin if I back out of selling an apartment at a previously agreed-upon price (630,000), after discovering that its current market price is much higher (700,000-750,000), especially since the agreement did not include contracts or a down payment, and I did not verify its market price before the agreement?

The conclusion of a contract occurs through offer and acceptance, along with the knowledge of the price and the item being sold. If these conditions are met, the sale is legally concluded and becomes binding on both parties, and it cannot be rescinded except through mutual cancellation (iqālah). If offer and acceptance have not occurred, then the sale has not yet been concluded. Nevertheless, if each of you has trusted the other and you have agreed on the price and the item, then you are not permitted to sell the item to someone who bids more than the first buyer's offer, due to the Prophet's (peace be upon him) saying: "A Muslim should not outbid his brother." As for withdrawing from completing the contract for a reason other than the appearance of a buyer at a higher price, it is permissible, though disliked, and does not obligate a financial right on the promisor unless the promisee incurs harm because of it.