Is it permissible to withdraw from purchasing an apartment after discovering that its actual area is less than what was agreed upon, knowing that the buyer does not wish to harm the seller?

There is no objection to retracting from the sale and revoking it if the agreed-upon description is missed and the item is less than what it was bought for. The buyer has the right to return it or keep it at a pro-rata price. So, one can keep it while taking the price difference, or return the apartment to its owner.