Bava Batra 104 - Contract for the Sale of Land
If one says to his fellow, "I am selling you a beit kor of earth as measured by a rope," it is understood that he means to sell precisely that amount of land - no less and no more. If the buyer later measured and found it to be less - the seller must refund him the missing amount.
Although in the sale of movable items a small deviation voids the purchase, the Sages assessed that in the case of real estate the buyer wants the sale to stand, even if the quantity is slightly more or less, as long as he is compensated for the difference. If the seller gave him more than a beit kor, the buyer has to return the excess land. If the seller said, "beit kor, be it less or more," the sale stands, provided that the discrepancy is no more than 1/24.
Art: Paul Gauguin - Breton Landscape Fields By The Sea