Gemara

(Mishnah #1 - Ben Nanas): If one said 'I sell to you (a Beis Kor) measured by a rope, be it less or more', (the latter clause) 'be it less or more' overrides 'measured by a rope'. If he said 'be it less or more, measured by a rope', 'measured by a rope' overrides 'be it less or more'.

(R. Aba bar Mamal): Other Tana'im argue with Ben Nanas.

Question: We already know this from a Mishnah!

(Mishnah #2): A case occurred in which one rented a bathhouse 'for 12 Dinarim for the year, a Dinar per month' (and it became a leap year). R. Shimon ben Gamliel and R. Yosi ruled that he pays half for the added month.

Answer: One might have thought that there, we are not sure if he retracted (and wants 13 if it will be a leap year), or if he only comes to explain (that rent is due every month), but here he surely retracted, and Chachamim would admit to Ben Nanas. R. Aba teaches that this is not so.

(Rav Yehudah citing Shmuel): Mishnah #1 is like Ben Nanas, but Chachamim say that the buyer has the lower hand (since we are unsure which clause overrides the other).

Question: 'This is like Ben Nanas' implies that the Halachah does not follow him. Rav and Shmuel taught that if one sold 'a Kor for 30 (Sela'im)', either can retract until all is measured out. If he said 'a Kor for 30, each Se'ah for a Sela', after each Se'ah is measured out, neither can retract. (The latter clause overrides the former. Each Se'ah is a sale by itself!)

Answer #1: 'This is like Ben Nanas', and the Halachah follows him.

Question: Shmuel taught that in Mishnah #2, they asked the Halachah in the middle of the month. If they ask at the beginning of the month, the renter must pay for it. If they ask at the end of the month, the renter need not pay! (The latter clause does not override the former.)

Answer #2: Really, Shmuel rules unlike Ben Nanas. Regarding the bathhouse, whoever is Muchzak has the upper hand.

(Rav Huna citing bei Rav): If Levi said 'I sell this for Istira (i.e. 96 Perutos), 100 Ma'os' (and Yehudah agreed), he owes 100 Ma'os. If he said 'I sell for 100 Ma'os, Istira', Yehudah owes Istira.

Question: Rav already taught elsewhere that the latter clause overrides. He obligates the renter to pay for the bathhouse in the added month!

Answer: One might have thought that regarding the bathhouse, the latter clause explains the former. (Therefore, he also taught about '100 Ma'os/Istira' to teach that the latter clause overrides the former.)

Bava Metzia 102b (Rav Nachman): The owner is Muchzak (in the bathhouse. Even if he claims at the end of the month, the renter must pay.)

Rav Nachman is unsure which clause we follow. Even if the clause favoring the renter was said last, the owner is Muchzak and the renter must pay.

Rishonim

Rif and Rosh (7:1): Shmuel is unsure which clause overrides. We do not take anyone's property when there is a Safek. The Halachah follows Shmuel against Rav in monetary laws.

Rif (ibid.): Rav Nachman argued about the bathhouse and said that even if they ask at the end of the month, the owner is paid. He does not hold that we follow the latter clause. He says so even if he rented 'for a Dinar per month, 12 Dinarim for the year'! Rather, he holds that land is in the owner's Chazakah. We do not take it from him without a proof. The same applies to 'measured by a rope, be it less or more' and '100 Ma'os, Istira.' We follow the lesser clause, for property is in the owner's Chazakah, and the other is ha'Motzi mi'Chavero Alav ha'Re'ayah. Rav Nachman and Shmuel agree to this. The Halachah follows them.

Rambam (Hilchos Mechirah 28:11): If one sold a Beis Kor, and said 'measured by a rope, be it less or more', or 'be it less or more, measured by a rope', we follow the lesser clause. He has only as if he said 'be it less or more.'

Ra'avad: I do not understand this. The lesser clause is what is lenient for the Nitva (defendant)! If the buyer gave money, he is the Tove'a (claimant). If he did not pay and made Chazakah on the land, and the seller demands more money, he is the Tove'a! Sometimes giving too little is a leniency for the Nitva, and sometimes it is a stringency. The same applies to giving too much.

Prishah (CM 218:9 DH veha'Ra'avad): The Ra'avad holds that if the buyer already paid, in every case he is the Tove'a. If too much land was given, the case is that he paid for the extra, and now claims asks to get back the extra he paid (for perhaps we follow 'measured by a rope').

Magid Mishneh: The Ri mi'Gash explains that the bearer of the document has the lower hand. He seeks to take from the other, so he must bring a proof. Therefore, he gets based on the lesser clause, be it first or last. The Rashbam says that the seller is Muchzak in the land, so he he has the upper hand. If he gave too litlle, he says 'be it less or more' (so I need not compensate you). If he gave too much, this is why he said 'measured by a rope.' The Rambam agrees. I say that even if the seller comes to take money (for the extra), we force the buyer to pay, for the seller was Muchzak in the land. We say similarly regarding contradictory clauses of rental.

Poskim

Shulchan Aruch (CM 218:14): If one sold a Beis Kor, and said 'measured by a rope, be it less or more', or vice-versa, we follow the lesser clause, and he has only as if he said 'be it less or more.'

SMA (42): 'The lesser clause' is the one that diminishes his words. He said that he sells a Beis Kor, and we say that it need not be a Beis Kor. It can be less or more. Alternatively, there are three laws that depend on three things that he said. 1) If he said Stam 'Beis Kor', he must give a full Beis Kor. He may not deduct from the money instead. 2) 'Measured by a rope' also connotes precision, but he may deduct (if he gave less). 3) 'Be it less or more' is a weak clause, for it means that he is imprecise. The Shulchan Aruch teaches that we follow it.

SMA (43): We follow 'be it less or more' whether this harms the buyer, i.e. he received too little, or whether it helps him, i.e. if he received extra. This is why he says 'he (the buyer) has only...' This is even if he did not pay yet and received too little. He must pay the full price of the Beis Kor, for the sale was finalized and the price is a debt.

Prishah and Drishah (9): The Tur explains that the Rambam holds that we follow the weakest clause. The Rashbam explains that we follow the clause that weakens the buyer. The Tur holds that the Rambam and Rashbam say so even if the buyer did not yet pay. The Rashbam holds that since the seller is Muchzak, he can say 'I said both clauses, for I was unsure if it is less or more. I said 'measured by a rope' in case it is more, and 'be it less or more' in case it is less. Since the buyer was quiet, he accepted this. The Rashbam said 'Chachamim hold that we follow the lesser clause; the case is, he received the money.' He does not mean that the buyer has the lower hand only if he already paid. The Rashbam said so to avoid a contradiction in Shmuel. However, regarding a bathhouse he rules unlike Shmuel; even if they ask at the end of the month, the owner is paid fully, for he is Muchzak, and the Safek began before the month began. The Beis Yosef says that we follow the lesser clause, for whoever is Muchzak in the money has the upper hand. This is like Shmuel. The Halachah is unlike him! The Magid Mishneh says that the Rambam is like the Rashbam. The Tur says that they disagree. This is correct. Also from the Ra'avad we infer that the Rambam holds that the buyer gains when extra was given. The Ra'avad argues, for 'the lesser clause' connotes that we are always lenient and weaken the Tove'a. Chachamim did not say who is weakened, for sometimes it is the buyer, and sometimes it is the seller. However, Perush ha'Mishnayos is like the Magid Mishneh. This requires investigation.

Rebuttal (Shach 2): The Magid Mishneh and Beis Yosef are correct. The Ir Shushan and Bach hold like them, and Perush ha'Mishnayos supports them. We cannot say that 'the lesser clause' diminishes 'Beis Kor.' The Mishnah does not mention Beis Kor! The Rambam just uses it to explain 'measured by a rope'! They mean like the Ra'avad says; the Ri mi'Gash and Rashbam agree with the Ra'avad l'Halachah. The Magid Mishneh says 'even if the seller comes to take money, we force the buyer to pay, for the seller was Muchzak in the land.' He does not means that we force him to buy if he does not want. Chazakah in the land does not help for this! Also, we force a renter to pay for the extra month only if he already used it! (If not, he can say that he will not use it.) Rather, if the buyer wants the land, he must buy and pay also for the excess. Until he does, the seller is Muchzak in the land. The Ra'avad's words connote that he disagrees, but I say that he agrees. The Ra'avad discusses when the seller wants the buyer to buy and pay for the excess. In this case, we do not force the buyer. Perhaps also the Magid Mishneh understood that the Ra'avad comes to explain the Rambam.