Explanation #1 (Rashbam): Rava said that any matter of (mistaken) measure, even less than the Shi'ur of Ona'ah, it is returned. This refers to Metaltelim.

Note: The Rashbam said so on the Mishnah (103b). There the Rashbam said that Rava means that the sale is Batel. It seems that this Tosfos cites our Mishnah (PF).

Rebuttal: Amidst his sharpness, he did not investigate. [Rava's teaching] is brought explicitly in Kidushin (42b) regarding land. It says that also regarding land, [Chachamim] taught [that an error less than a sixth is pardoned] only regarding value, but not regarding area, like Rava taught...!

Explanation #2 (Ri): Rava discusses when they measured and erred about the measure. However, here they did not measure yet, but they come to measure, and also from the beginning he knew that it is not so exact that there will not be less or more. (Therefore, the sale is not Batel. Rashash says that the same applies if they measured, but they did not deceive him.)

Question: He should say to him "I did not buy with intent for this" (a field smaller than you said)!

Answer: The case is, he was in the field, like I explained. From the beginning he knew that it is not exact.

Explanation: The Reisha does not contradict the Seifa, for one of them is not precise (we may not infer from it). [The Reisha] mentioned (when he said one expression, "measured with a rope" or "be it more or less") only due to the Seifa, like the Rashbam explained.

Question: [The Gemara] learns "be it more or less" from "like a Beis Kor." [Really,] "be it more or less" connotes more that the sale stands than "like a Beis Kor" connotes!

Answer: When he said to him "I sell to you a Beis Kor of earth, be it more or less", we can say that his first words are primary. Therefore it says that we learn from "like a Beis Kor."

Observation: In several places we are meticulous to ask this, and in several places we are not meticulous.

Question: Since the Tana taught the Reisha "a Beis Kor of earth" Stam, that the sale stands, why does the Seifa need to teach "be it more or less"? All the more so [the sale stands]!

Answer #1: It teaches that even so, only [if the error is at most] a Beis Rova [the sale stands], but not if it is more than a Rova.

Answer #2: He taught the Seifa to reveal about the Reisha, lest we say that in the Reisha he said "be it more or less." The Seifa teaches "be it more or less", so we infer that in the Reisha, he did not say so. We find like this in several places.

Question: Above, R. Yitzchak said that rocky land [that is measured with the field is up to] four Kabim, but if it is more, it is not measured with it;

Here [the Beraisa] says that a Beis Kor Stam is as if he said "be it more or less", and even if it is not rocky at all, but the land was deficient [a Rova per Se'ah, i.e. seven and a half Kabim], we would say that the sale stands, and all the more so now [that the measure is full, just four Kabim were rocky]!

Answer #1: R. Yitzchak said only to join [up to four Kabim of rocky land] to reduce from [a deficiency of] a Rova per Se'ah. (Indeed, if there is no deficiency, up to a Rova per Se'ah may be rocky, for even if it were lacking, the sale would stand!)

Answer #2 (Rashba): This is not difficult at all. Here we discuss when he stands in the field and sees how much there is;

However, when he does not see it, obviously there we do not say that the sale stands. If he stipulated to sell to him a Beis Kor of earth, can he give to him a Beis Kor lacking a Rova per Se'ah?!

It is reasonable that there, even through deducting, the buyer would not receive, for he says to him "I did not buy with intent for this"! He intended to receive a full Beis Kor. The Reisha of our Mishnah discusses this.

This is why R. Yitzchak said that that rocky land [that is measured with the field is up to] four Kabim, but if it is more, no;

If we would view them as if they do not exist at all, even if he were missing only a Kav, he would be obligated to complete [the Beis Kor], for he bought a full Beis Kor.