He bought an apartment on the basis that it would be a certain area (square footage), and it was built larger. Does he have to pay the difference?
If a person sells land or a house on the basis that it is a certain area, then it turns out to be more than what was estimated, the sale is valid and the one who was unaware of the additional area and it does not suit him, has the option to go ahead with the transaction or cancel it.
It says in Zaad al-Mustaqni‘: If he sells the house on the basis that it is ten cubits, and it turns out to be more or less, the transaction is valid and the one who was unaware and it does not suit him has the option (of going ahead or cancelling).
In this case the seller cannot be told to take the extra area which is approximately twenty square metres. Hence we say that he has the choice of going ahead with the sale or cancelling it.
If the buyer does not have the money to pay for the extra area or does not need it, he also has the choice and he is not obliged to buy the extra area, because it was not covered in the contract.
To sum up: both of you have the right to cancel, unlessas the seller decided to give you the additional area for free, in which case you do not have the choice, because you are not losing anything.
If he insists on being paid the price of the additional area, then you may either cancel the sale or agree on the amount for the additional area, and working out an agreement on the price is better.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If you both agree on what has been subtracted or added, that is permissible, because you both have the right. If you agree to waive it, such as if he says, ‘I sold it to you on the basis that it was 100 square meters, and it turned out to be 90 square meters, and you both reach an agreement to waive such and such of the price and agree to that, there is nothing wrong with it.
End quote from al-Sharh al-Mumti‘, 8/259
And Allah knows best.