Is it permissible to withdraw from the sale of a commercial shop, after an agreement has been reached and a portion of the price has been received, due to discovering that it was sold at a low price, or because there is an unknown additional area in the shop, or simply due to disliking the buyer's character, given that the period for completing payment was not precisely determined?

We thank the questioner for their eagerness and acceptance of the truth; this is the characteristic of a sincere believer, as stated in Surat An-Nisa, verse 65. The sales contract is valid as long as the payment deadline is known. الغبن (deception/fraud) necessitates the option (to revoke the contract) under its conditions. As for the increase in area, the increase belongs to you, and there is a difference of opinion regarding its effect on the validity of the sale. Some consider the sale void, while others consider it valid and the increase belonging to the seller. Ibn Uthaymeen holds that the sale is valid in both cases (increase or decrease), with the increase belonging to the seller and the decrease being the seller's responsibility. The option (to revoke) rests with the one who was ignorant of the amount and whose purpose was unfulfilled. Ignorance and unfulfilled purpose are conditions for this option. If the buyer is content with the increase for free, or the seller with a reduction in price, there is no option. In summary: you have the right to annul the contract if the buyer refuses to pay for the increase, or if you leave the increase to yourselves and cannot benefit from it after the sale, to prevent harm. As for the neighbor's dislike of the buyer's character, it has no effect on the contract.