We are learning the fourth Mishnah in the seventh chapter of Tractate Bava Basra, the last in this short chapter. This Mishnah also deals with the question of how to interpret a person's language in a sales agreement, and it contains two cases.
The First Case - "I Am Selling You Half a Field":
The Mishnah begins: "Ha'omer lachaveiro chatzi sadeh ani mocher lach" - if one says to his friend, "I am selling you half a field" - the parties finalized the transaction and performed a kinyan, but they did not specify in the agreement which half is being sold and how it is measured. The discussion does not revolve around the location of the half (the northern or southern half and the like), since a specific location was not mentioned, but rather on the method of measurement itself, for which there are two possibilities:
Measuring by area: Fifty percent of the land's area.
Measuring by value: Land whose value reaches half the value of the entire field, since there are parts of the field that are worth less because they are not fertile or because there are rocks in them.
The Mishnah decides: "Meshamnin beineihen venotel chatzi sadehu" - they appraise it between them and he takes half his field. According to the Rambam, "meshamnin beineihen" means that they conduct an appraisal: they evaluate the value of the entire land as it was before the division, and if it is found to be worth two thousand, the seller must give the buyer land worth one thousand.
Presumably, more than fifty percent of the land's area will actually pass into the buyer's hands. For if a person had to choose between a small field and a large field that yield equal crops, he would prefer the smaller one, because a small field requires less work in plowing, planting, and watering. Thus, a small, fertile field is more desirable than a large, less productive field. Therefore, the buyer will receive more than fifty percent of the land, but exactly fifty percent of the value. This is the novel teaching of the first part of the Mishnah: the division is made according to value and not according to area.
The Second Case - "I Am Selling You Its Southern Half":
If he said to him: "Chatziyah badarom ani mocher lach" - "I am selling you its southern half" - here, too, his words can be interpreted in two ways: half in terms of area or half in terms of value, and it is even possible that he does not literally mean the southern half, but rather the value of the southern half. To this the Mishnah says: "Meshamnin beineihen venotel chatziyah badarom" - they appraise it between them and he takes its southern half. First they evaluate the value, and then they give the buyer the southern half.
The view of the Rambam, and the halachah: Here, too, the division is made according to value and not according to area. They appraise the value of the entire field, and let us assume it amounted to two thousand. They then measure in the southern part of the field how much land must be given so that it reaches a value of one thousand, half the value, and this is the buyer's portion.
The view of the Bartenura, which is not the halachah: Its law is the same as the first part of the Mishnah: they measure the southern half according to area, check what its value is, and this is the value the seller must give. However, the seller is permitted to give this land from anywhere in the field, from the north, from the east, or from the west; the mention of the south was intended solely to determine the value.
The Place of the Fence and the Trenches:
The Mishnah adds a detail that completes the picture regarding the division of the field. As we saw in the first chapter, they used to partition between two parts of a field with a wall, so that animals would not cross from one person's portion to his friend's portion. Here we learn that they did not suffice with erecting a wall, but they dug two trenches next to it, one large and one small, which would prevent the animals from jumping over it.
The seller expects the buyer to have the trenches on his side, so that the animals will not cross from the new buyer's field and invade the seller's field. The question is who allocates this area, and about this the Mishnah says: "Vehu mekabel alav mekom hagader hecharitz uven charitz" - and he accepts upon himself the place of the fence, the trench, and the small trench. The buyer accepts upon himself the place of the fence, the charitz, which is the trench next to the fence, and the ben charitz, the small trench next to it.
And what are their dimensions? The Mishnah details: "Vechamah hu charitz" - how wide is a ditch? Six tefachim, "uven charitz" - and a small ditch? Three:
The charitz: Its width is six tefachim, the size of an amah (its depth is not specified here).
Ben charitz: Its width is three tefachim.
The space between them: One tefach separating the two, next to the wall.
It turns out that a total of ten tefachim of space is allocated next to the wall.
Regarding the question of who allocates this space, the Rishonim disagree:
The opinion of most Rishonim: The entire area is allocated from the purchased field. This means the buyer receives fifty percent of the field minus the space needed to build the wall and dig the two ditches.
The opinion of the Rashbam on the Gemara: The place for the fence is allocated from the seller's side, while the ditches are allocated from the buyer's side.
For the halachah, the Mechaber rules in the Shulchan Aruch like the first opinion, that everything comes from the buyer. This is because the seller is the muchzak (the one in possession) and he is the one interpreting his own words, and therefore he can force the buyer to allocate the place for the wall and the two ditches from his own portion. The Rema rules like the opinion of the Rashbam, that the place for the wall is allocated from the seller's side.
In summary: In this Mishnah, we learned that if one says, "I am selling you half a field" - they evaluate it between them and divide it by value and not by area, and therefore the buyer will actually receive more than half the land area. This also applies when one says, "Its southern half," which according to the Rambam and the halachah is measured in the southern part according to the value of half the field, while according to the Bartenura only the value is determined and it can be given from anywhere in the field. We also learned the law regarding the place of the fence, the charitz, and the ben charitz and their dimensions, as well as the dispute between the Mechaber and the Rema regarding who allocates the place for the wall.