Bava Basra, Chapter 7, Mishnah 2. The Mishnah discusses the question of what the law is when the amount agreed upon by the parties does not perfectly match the actual size of the land being sold. The problem that arises here is this: The seller speaks of land the size of a beis kor - let us say, five acres - but upon measuring, it turns out that the area is 4.99 acres or 5.01 acres. Returning that tiny difference of 0.01 acres to the seller, or refraining from purchasing it, leaves a tiny, unusable strip of land in the hands of the seller. He cannot plant such a small area, and it makes no economic sense.
Summary of the Mishnah's ruling:
Less than the amount: If the area is slightly smaller than agreed upon, and beyond a certain threshold, the price must be reduced.
More than the amount: If the area is slightly larger, the choice is given to the seller, and he has the advantage. He may take back that small strip, or say to the buyer: I am not taking this tiny strip, you must take it with you and pay a proportional addition to the price for it.
Therefore, the Mishnah defines the acceptable limits - from what amount and onwards these rules take effect.
The first case - Measuring with a rope:
"Beis kor afar ani mocher lecha" - As in the language of the previous Mishnah. Reuven is selling Shimon a beis kor of land, and "afar" means that this is the exact amount he is to receive. "Midah bechevel" - Exact measurement: When he says a beis kor, he means exactly one point zero zero zero beis kor, and therefore the area must be measured.
"Pichas kol shehu - yenakeh" - If upon measuring it turns out that there is no complete beis kor here but only zero point nine nine, the deficit must be restored: one percent is deducted from the price, and the money returns to the buyer.
"Hosir kol shehu - yachazir" - And what is the law when the area is slightly larger than agreed upon, such as one point zero one beis kor? The term "yachazir" usually means returning, but in our context the meaning is that the buyer gives additional money - ironically enough - so that the seller will not be left with a tiny strip of land. This will be explained further below.
The second case - "Hen chaser hen yeseir":
"Im amar hen chaser hen yeseir" - That is, an exact measurement was not stated, but rather a sale of more or less a beis kor. And in the language of the Mishnah: "Afilu pichas rova l'seah o hosir rova l'seah - higi'o". A rova per seah is one twenty-fourth of a beis kor, meaning about four point two percent. The meaning is this: If it was stated that the sale is an approximation, and the measurement deviation is less than four point two percent, the deal is closed. They said approximately, and so it indeed turned out.
"Yeseir mikan - ya'aseh cheshbon" - If the deviation is greater than this amount, the measurement is no longer close enough, and a calculation must be made to adjust as necessary.
"Mah hu machazir lo? Ma'os" - That is, what must the buyer return to the seller? Money. For example: If the permitted range of deviation is four point two percent, and it turns out that the area is one point zero five beis kor, five percent larger than estimated, the buyer must add five percent to the purchase price and receive the additional land. For the alternative is to force the seller to take back a small strip of land that has no real use on its own, and he will be left with a useless plot - and this is not fair.
"Ve'im ratzah - machzir lo karka" - the choice is up to the seller. If he wishes, he can say to the buyer: I did not realize the area was so large, and therefore I will take the extra strip for myself; you will be left with exactly one point zero zero zero beis kor, and I will keep the addition for my own use, if only to grow a palm tree on it.
"Velamah amru machzir lo ma'os? Leyapos kocho shel mocher" - why is the choice given to the seller to demand additional money, rather than canceling the transaction or setting it at the exact measurement and being left with a small strip of land? Because we want to give the seller the upper hand, for the reason explained: we do not want the seller to be left with a useless plot of land.
When the excess must be returned specifically as land:
All this applies when the excess strip is small, but once the excess is large enough to justify independent use, the law changes. In the first chapter of the tractate we discussed the measure of land suitable for use as a field, which is subject to the law of division, and the measure there is nine kavin:
In a field: An area of nine kavin, which is a se'ah and a half (in a beis kor there are one hundred and eighty kavin). However, at this stage it is no longer a question of percentages, but an actual area called an 'area of nine kavin' - about three thousand seven hundred and fifty square amos, which is about one thousand square meters, a dunam, an area approaching the size of an Olympic swimming pool. Such an area is suitable for use as a field.
In a garden: When the land is not used for sowing grain but for growing vegetables, the measure is an area of half a kav - one three-hundred-and-sixtieth of a beis kor, but again, this is an actual measurement and not a relative one: about fifty square meters, about the size of four parking spaces.
According to Rabbi Akiva: A beis rova - a quarter of a kav, about half of the previous measure: about twenty-five square meters, five by five meters, about two parking spaces. Such an area is already useful as a vegetable garden.
If this is the situation - he returns the land to him. This amount of land is useful in its own right, and therefore the seller keeps the extra land for himself and cannot demand additional payment. The buyer gets exactly what he bought - a beis se'ah, and not a beis kor - and pays exactly what he agreed upon.
"Velo es harova bilvad machzir, ela es kol hamosar" - the Mishnah clarifies that once a correction is made, the margin of error that existed beforehand is not left in place. Even though the sale was stated with the term 'about a beis kor,' and therefore a deviation of four point two percent was permitted, we do not only correct what goes beyond this measure. When making a correction - we correct it down to the exact square centimeter, as much as can be measured. The rest is returned to the seller, and the buyer receives exactly what was agreed upon, in the most precise measurement we have.