Gemara

77b (Mishnah): If one sold a yoke, he did not sell the oxen.

R. Yehudah says, the price indicates what he sold.

The case is, people call a yoke a yoke, and call oxen oxen, and some use 'yoke' to refer to the oxen as well. R. Yehudah holds that the price indicates what he sold. Chachamim hold that the price is not a proof.

92a (Mishnah): If David bought Moshe's Peros and planted them, and they did not grow, even if he bought flax seeds, Moshe is exempt;

R. Shimon ben Gamliel says, if he bought seeds that are not (normally) eaten, Moshe is liable.

(Rav): If Reuven sold an ox to Shimon, and it was found to be a gorer, it is a Mekach Ta'os (a mistaken sale; it is void).

(Shmuel): Reuven can say 'I sold it to you for slaughter.'

Question: Let us see whether Shimon buys oxen for plowing or for slaughter!

Answer: The case is, he buys for both.

Question: The price should show for what he bought!

Answer: The case is, the price is the same for both.

Question: If so, in either case he gets back what he paid!

Answer: If it is valid, Shimon must toil to sell it. If it is invalid, he can demand that Reuven refund the money.

Question: If Reuven has no money to repay Shimon what he owes, he may give to him the ox!

Answer: The case is, Reuven has other money. Rav says that the sale is invalid. We follow the majority, and most people buy oxen for plowing. Shmuel says, he can say 'I sold it to you for slaughter.' We follow the majority for Isurim, but not in monetary matters.

Rishonim

Rif and Rosh (6:2): The Halachah follows Shmuel.

Nimukei Yosef (DH Iy): If Reuven has money, he must return what Shimon paid, like a borrower. If he has no money but has Metaltelim, he must give the Metaltelim, for they are in place of money. If he claims that he has no money or Metaltelim and seeks to pay with land, the Ge'onim enacted a Cherem (to take effect if he is lying).

Rambam (Hilchos Mechirah 16:5): If Reuven sold an ox to Shimon, and it was found to be a gorer, he can say 'I sold it to you for Shechitah.' This is when Shimon buys for plowing and for Shechitah. If Reuven knew that Shimon buys only for plowing, it is a Mekach Ta'os. The same applies to all similar cases.

Rosh (6:2): If the buyer buys for plowing, and the seller knew this, he sold for plowing. Chachamim agree that the price is a proof when it does not contradict the primary way people speak.

Rosh (Bava Kama 5:1): If the buyer normally buys only for one of them, we assume that he bought like he usually does.

Piplulei Charifta (2): Here the Rosh connotes that even if the seller did not know for what the buyer normally buys, we follow his normal practice and it is a Mekach Ta'os. In Bava Basra the Rosh says that it is a Mekach Ta'os only if the seller knew the buyer's normal practice. I say that when he does not know the buyer's practice, the buyer needs to inform him. If he did not inform him, it is as if he abandoned his practice and buys for both.

Tosfos (92a DH v'LIchzi): Chachamim hold that the price is not a proof only when the majority (of how people speak) and Chazakah (he has the money) favor the seller. Even according to texts that say (on 77b) 'some call a yoke a yoke... and some...' (there is no majority), the price is not a proof because the Chazakah favors the seller. If there is a majority (e.g. most buy cows for plowing) against a Chazakah, the price determines what was sold.

Rashbam (92a DH v'LIchzi): Chachamim hold that the price is not a proof only when not everyone calls oxen 'yoke'. The seller can say that the buyer calls oxen oxen, and he sold only a yoke. Ha'Motzi mi'Chavero Alav ha'Re'ayah; since the buyer cannot prove how he speaks, he gets only a yoke. Here, we know that the buyer ofen buys for plowing and Shechitah, therefore, the price is a proof. Also above, if everyone calls also oxen 'yoke', even if they sometimes call them oxen, the price is a proof, for we know that the buyer often calls them yoke.

Tosfos (92b DH Iy): The Rashbam rejects the text that says 'if Reuven does not have those coins (that he received from Shimon)...', for if he has any money, he must pay back with money, like a borrower. The Ri defends the text. A borrower is different. He lent money with intent to get back money. When a seller owes due to a Mekach Ta'os, if he spent the money he is like a damager (and he need not pay with money, even if he has other money). R. Chayim Kohen says that even a borrower need not pay with money. He has no more obligation than a damager, who (when he pays with land) gives Idis (highest quality land). All Metaltelim are like Idis (highest quality land), for they can be sold anywhere. R. Tam says that a borrower with money must pay money. If he has no money, we tell him to sell his property and pay money. A damager may pay with anything, even if he has money.

Poskim

Shulchan Aruch (CM 232:23): If Reuven sold an ox to Shimon, and it was found to be a gorer, if we cannot prove whether or not he bought it for plowing or for slaughter, e.g. Shimon buys for both of them, and we cannot bring a proof from the price, e.g. the prices are the same, the sale stands. Reuven can say 'I sold it to you for slaughter.'

SMA (57,59): One may not keep a gorer alive. Shimon slaughters it, sells the meat and uses the money to buy an ox for plowing.

Yam Shel Shlomo (Bava Kama 5:3): The price is a proof when the prices for plowing and for Shechitah differ greatly.

Yam Shel Shlomo (Bava Kama 3:1): The Gemara said that Kad and Chavis (big and small barrels) are interchangeable. I.e., whoever is Muchzak in the money keeps it, even if the majority uses the words oppositely to the way he does. This is when the price is not a proof, e.g. the prices are about the same. If there is a sizable difference, the price is a proof and favors the Muchzak even if the majority opposes him, or the price favors the one who holds like the majority even if he is not Muchzak.

Shulchan Aruch (ibid): Even if most buy for plowing, we do not follow the majority to make Reuven pay. If Shimon still has the money, ha'Motzi mi'Chavero Alav ha'Re'ayah. This is not only if most buy for plowing, rather, even if half buy for Shechitah.

Gra (31): We do not follow the majority only when the Chazakah favors the seller, for then ha'Motzi mi'Chavero Alav ha'Re'ayah.

Prishah (18): We require a majority with the Chazakah only regarding seeds, for the buyer does not return them. One returns a goring ox, therefore, the Ramah does not require

Bach (18): Even if only half buy for seeding, we follow the Chazakah of the money, and the buyer need not pay.

Bach (21): Tosfos (Sanhedrin 3b DH Dinei) says that we follow the majority even for capital cases, all the more so we should follow it for monetary cases! We must say that the majority without buy for plowing is weaker than other majorities, therefore, it does not help for money. We also do not follow the majority who buy seeds for planting! It seems that the majority of oxen or seeds are bought (by a few buyers) for plowing and planting, but the majority of buyers buy (small amounts for) eating.

Dagul me'Revavah: The Shulchan Aruch connotes that if most buy for Shechitah, even if the buyer is Muchzak, he must pay. However, we do not follow the majority in monetary laws! Like the Bach, we can say that a majority who buy for Shechitah is a proper majority, and we follow it.

Shulchan Aruch (ibid): If Reuven knows that Shimon normally buys only for plowing, it is a Mekach Ta'os.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): It seems that the Tur holds that if Reuven did not know this, the sale stands.

Shulchan Aruch (ibid): If he normally buys only for Shechitah, presumably he bought for Shechitah. If he normally buys for both, and it is clear from the price that he paid for a plolwing ox, it is a Mekach Ta'os. If he paid the price of an ox for Shechitah, he bought it for Shechitah.

Prishah (Sof ha'Siman): According to the Rashbam, if the prices differ, this proves for what he bought it, if everyone calls a plowing ox or an ox for Shechitah (just) 'ox'. According to the Ramah, it suffices if most people call a plowing ox (just) 'ox' (even if some always specify).

Rema: Whenever it is a Mekach Ta'os and Reuven must return the money, if he has money, he must return money, for he is like a borrower. This is unlike the opinion that he may pay with land.