Gemara

Question: If it (a Chatzer, field...) is too small for each partner to get a proper share, what is the law?

Answer #1 (Rav Yehudah): 'Gud O Agud' applies. (A partner can set a price and say 'buy my share for this price, or I will buy your share for this price.')

Answer #2 (Rav Nachman): 'Gud O Agud' does not apply.

Question (against Rav Yehudah - Mishnah - Beis Hillel): A half-slave alternates. He serves his master for one day, and himself for one day;

Beis Shamai: If so the slave cannot marry at all! Rather, the master must free him. The slave owes to the master half of his value, and writes a document. (Beis Hillel later agreed, but only due to Beis Shamai's reason. If not, a half-slave could not force his master to set a price for the slave half to redeem himself!)

Answer: Gud O Agud does not apply there, for the slave can buy, but he cannot sell (his free half, for a half slave cannot become a full slave).

Question (against Rav Yehudah - Mishnah): If Reuven is rich, and his brother Shimon is poor, and they inherited a bathhouse or an olivepress that the father made for his own use, Reuven can say 'also we will use them ourselves. Go buy slaves to heat it up, and go buy olives and press them!' (Shimon cannot. Reuven will use it himself; Shimon cannot tell Reuven to buy his share!)

Answer: There, Shimon cannot (afford to) buy Reuven's share.

(Mishnah): Even if both want to divide a Sefer, they may not.

(Shmuel): This is only if the Seforim are bound together. If each is bound by itself, they divide.

Abaye: Shmuel holds like Rav Yehudah. If Gud O Agud did not apply, in any case either could stop the other from dividing!

Rejection (Rav Salman): Shmuel teaches that the Mishnah forbids dividing Seforim wrapped together even if both want to divide. (He did not address whether or not Gud O Agud applies when only one wants to divide.)

(Ameimar): The Halachah is, we say Gud O Agud.

Rav Ashi: Rav Nachman argues!

(Ameimar): I argue with Rav Nachman.

Question: Chinena left two slaves to his two sons. One knew how to cook, and the other knew to sew. Rava ruled that Gud O Agud does not apply

Answer: That case is different, because each of them needs both slaves. Saying 'take one' is not Gud O Agud.

Question: Regarding Seforim, both need both, yet Shmuel taught that if each Sefer is bound by itself, they divide!

Answer: Rav Salman said that this is when both want to divide.

Rishonim

Rif: Some learn from the case of brothers who inherited a bathhouse or olivepress that we say Gud O Agud only when either could buy, but not when one is rich and one is poor. I disagree. When one says Gud O Agud, it does not matter if the other could buy. In the Beraisa, the Oni said Gud O Agud, and he could not buy. Therefore, the Ashir can say 'I do not want to buy. You buy!' Since the Oni cannot buy, Gud O Agud is Batel. The Halachah is, we say Gud O Agud.

Rambam (Hilchos Shechenim 1:2): When the law is that something need not be divided, or it cannot be divided, e.g. a slave or Kli, if David said to his partner Levi 'sell to me your share for this amount, or buy my share for this amount', we force Levi to buy or let David buy it. If David cannot buy it or find someone to buy it., he cannot force Levi to buy, even at the cheap price. Levi can say 'I do not want to buy, rather, to sell.' Therefore, if an Ashir and Oni inherited a bathhouse or olivepress that the father made for his own use, the Ashir can say 'go buy olives and press them, or go buy slaves to bathe in the bathhouse.' The Oni cannot force him to buy his share, unless he says 'buy from me, or sell to me, and I will borrow and buy, or sell to others.'

Beis Yosef (CM 171 DH u'Mah she'Chosav Rabeinu v'Ein): The Tur says that the Oni's only solution is to sell his share to the Ashir, or give his share to the Ashir, for the Ashir can say 'Gud O Agud, or use it.' Perhaps he argues with the Rambam, rather, like Teshuvas ha'Rosh, who says that Gud O Agud is only when one buys for his own use, but one who sells his share cannot say Gud O Agud and force his partner. Perhaps the Rosh does not argue; the Tur says that the Oni has no other solution because it is hard to find one who will buy his share to harm the Ashir.

Rosh (1:50): Rashi connotes that one may set a price more than the true value. Some say that Beis Din assesses it, and whoever wants to pay gets it, like we say in Kesuvos (91a) 'the orphans may not assess the property for more than its worth (to enable the enactment that the sons of each wife inherit their mother's Kesuvah). Rather, Beis Din assesses it.' This is wrong. There, saying so would cause a loss to the sons of the other wife. Here, there is no loss. Some Ge'onim said so. The words of the Gemara 'Ma'alin Oso b'Damim' connotes (that one may inflate the price,) like Rashi. The Rambam says similarly. This is primary. Gud O Agud applies to the entire Mishnah except for a Chatzer, for it is not an entity unto itself. It serves the house; every house needs a Chatzer.

Poskim

Shulchan Aruch (CM 171:6): If something need not or cannot be divided, e.g. a slave or Kli, if David told his partner Levi 'sell to me your share for this amount, or buy my share for this amount, Levi must buy or let David buy it.

Beis Yosef (DH Ela): We hold like Rav Yehudah, for Ameimar rules like him and it seems that Rav Ashi agrees.

Beis Yosef (DH uvi'Teshuvos): Rashi explains Gud O Agud to mean 'fix a price for me to buy or sell; if not, I will fix a price for me you to buy or sell. The Rambam holds that it means 'I will fix a price. Sell it to me for this, or I will sell my share for this.'

Rema: Even if he said to give much more than its value, Levi must buy or sell.

Beis Yosef (DH v'Chosvu): The Ri says so. The Ritzva disagrees, for if so one could force his partner to split a Chatzer that need not be split.

Shulchan Aruch (ibid): If David does not want or cannot buy it or find someone to buy it, he cannot force Levi to buy, even at the cheap price. Levi can say 'I do not want to buy, rather, to sell.'

Gra (22): In each case, the Gemara said 'he can buy, but he cannot sell' (or vice-versa) from the perspective of the one who demanded Gud O Agud. This shows that it depends on his ability to do either.

Shulchan Aruch (ibid): Therefore, if an Ashir and Oni inherited a bathhouse or olivepress that the father made for his own use, the Ashir can say 'go buy olives and press them, or go buy slaves to bathe in the bathhouse.' The Oni cannot force him to buy his share, unless he says 'buy from me, or sell to me, and I will borrow and buy, or sell to others.'

SMA (13): If both are rich and neither wants Gud O Agud and they do not want to use it together, they alternate using it for a period of time.

SMA (14): All the more so, the Oni can sell his share to an Ashir who will be in place of the Oni. The buyer may use it with the rich brother or alternate times, and can later say Gud O Agud if he wants.

Gra (23): We learn that he may borrow from the slave, who writes a document obligating himself to pay half his value.

Rema: Some say that one cannot say Gud O Agud when he does not have money, and must sell his share.

SMA (14,15): Even this opinion agrees that if the Oni sold his share to an Ashir, later the Ashir can say Gud O Agud. It disallows only to say 'if you will not buy, I will sell it for what I can, and give to you your share.'

Bach (27): The Tur says that Teshuvas ha'Rosh holds that Gud O Agud is only when he wants it for his own use. This is unlike all the Ge'onim, who allow even when he will borrow to buy, or sell his share to others. The Rambam says so explicitly. The Rosh has no source to distinguish. He says that if not, the Ashir could not tell the Oni 'go buy slaves...' This is no proof. The Ashir can say so only when the Oni cannot find one to buy his share! I think that the Rosh agrees that if the Oni cannot find a buyer and will have a loss, he can say 'buy it for its value, or I will sell it for its value and give to you half.' The Rosh discusses only when he could sell his half for its value or a little less. He forbids only when the Oni asks more than its value. This is permitted only if he is ready to buy it for his own use. The Tur agrees with the Rosh's law, even though his proof was invalid.

Shach (7): The SMA's Perush is better than the Bach's. Agudas Ezov had difficulty because he understood the Rema differently.

Rema: One cannot say Gud O Agud for less than the value, for if so an Ashir could force an Oni to sell for less than the value.