(Mishnah): If a seller said to a buyer, "I sell to you a Beis Kor, measured by a rope'', if it is any amount less, the buyer deducts from the price. If it is any amount more, the buyer returns the extra (land or money).

If he said, "whether it is less or more'', even if there is a Rova too little or too much per Se'ah, there is no compensation.

If the error is more than this, he must make a calculation and return the money.

If the seller wishes, he can return the land.

Why then did Chazal say return the money? To benefit the seller (as he cannot use such a piece of land).

If the extra is Beis Nine Kavim in a field, or half a Kav in a garden, or according to R. Akiva a Beis Rova, (all of which being a significant amount and considered their own field) the land can be returned to the seller.

Even though if it had been exactly a Rova too little or too much, it could not be returned, but if the difference was more significant, he must return the entire amount, not just the addition to a Rova.

(Gemara) - question: (The Mishnah seems to contradict itself) - here (in the earlier clause) the Mishnah seemed to rule that up until a Rova of a seah, the sale is valid and only the amount that is beyond a Rova must be returned; but in the latter clause, the Mishnah seemed to rule that the entire amount must be returned?

Answer: Since it is more than a Rova, it has negated the words that he said of, "whether it is less or more'' and it is as if he has said that he is selling it "measured by rope'', so he must return it.