Gemara

Chachamim hold that if one overpaid a sixth, only the buyer can void the sale. If the Ona'ah more than a sixth, either one can void it.

(Rava): The Halachah is, if the Ona'ah (undercharge or overcharge) is more than a sixth, the sale is void. If it is exactly a sixth, the Ona'ah is returned.

Support (Beraisa - R. Noson): If the Ona'ah is more than a sixth, the sale is void. If it is exactly a sixth, the Ona'ah is returned;

Rebbi says, the seller (or buyer, whoever was deceived) has the upper hand. He can void the sale, or demand payment of the Ona'ah.

51a (Mishnah): The victim has the upper hand. (E.g. if it was the buyer,) he can demand back his money, or to be paid back the Ona'ah.

Bava Basra 83b (Rav Chisda): If Levi sold to David an item worth five (e.g. Zuz) for six, and its value rose to eight, David can retract, but Levi cannot, since David was overcharged. David can say 'had you not overcharged me, you could not retract. Now that you overcharged me, can you retract?!' If Levi sold an item worth six for five, and its value decreased to three, Levi can retract, not David, since Levi was underpaid. He can say 'had you not underpaid me, you could not retract. Now that you underpaid me, can you retract?!'

Support (Mishnah): If Reuven sold bad wheat, and it was found to be good, Reuven can retract.

Question: We already learn Rav Chisda's law from the Mishnah!

Answer: One might have thought that either can retract in Rav Chisda's case (the price changed). The Mishnah teaches that the buyer can retract. One might have thought that "Ra Ra Yomar ha'Koneh" (the buyer did not err; he always claims that the merchandise is poor).

Rishonim

Rif (Bava Basra 42a): A Ga'on says that Rav Chisda discusses accepting Mi she'Pora. I disagree. Rav Chisda is like the opinion that the victim of Ona'ah has the upper hand. We hold like Rava, who says that the sale is valid, and the Ona'ah is returned. However, we hold like Rav Chisda regarding Bitul Mekach. If one sold an item worth four for five, and before there was time to show it to a merchant or relative, the price rose to seven, only the buyer can retract. He can say 'had you not overcharged me... Now that you overcharged me, can you retract?!' Similarly, if one sold an item worth five for four, and the price dropped to three, only the seller can retract. He can say '... should you be able to retract because you underpaid me?!' The Gemara rejected the proof from the Mishnah for Rav Chisda, we do not reject his teaching, for his reasoning is sound.

Rosh (Bava Basra 5:14): Rav Chisda holds like the Stam Mishnah in Bava Metzia 51a. The Halachah does not follow him. We hold like Rava, who rules like R. Noson, that if the Ona'ah is exactly a sixth, the sale is valid and the Ona'ah is returned. However, we hold like Rav Chisda's primary Chidush: a seller cannot retract when the item later rose in value. Without Rav Chisda's teaching, one might have thought that even the seller can retract. All agree that one who overcharged cannot retract. One might have thought that here is different, for the price rose to the Shi'ur of Bitul Mekach regarding the seller before the sale was solidified, i.e. the time to show the item to a merchant or relative. Rav Chisda refutes this, for the buyer can say 'had you not overcharged me...' This does not apply to us, for we hold that if Ona'ah was a sixth, the sale stands. However, the Rif says that it applies to Bitul Mekach, e.g. one sold an item worth four for five, and before there was time to show it, the price rose to seven. When there is Bitul Mekach, either party can retract (Bava Metzia 50b). This is when the price was very far from the value, more than people err about. It is not called a sale at all. The Rivam says that when the Ona'ah is more than a sixth, the swindler can retract only if the victim claimed the Ona'ah. If he does not, he can say 'had you not swindled me...' If we would not say so, the swindler would have more rights when the Ona'ah was less than a sixth than when it is a sixth. Even though also now, when the Ona'ah is a sixth he returns it and cannot void the sale, this is reasonable, for this is not like a Bitul Mekach. When it is more than a sixth, he cannot retract if the victim pardons the Ona'ah, for Meshichah was done. R. Yonah challenged this, for the Gemara said Stam that when the Ona'ah is more than a sixth, either can retract. This cannot be when the victim claimed Ona'ah, for one cannot claim Ona'ah above a sixth, only Bitul Mekach! Also, claiming Ona'ah shows that he wants the sale to stand. It is unreasonable that the swindler can void the sale then. R. Yonah says that both can retract as long as the victim can retract. The sale is destined to be void, for the victim cannot claim the Ona'ah, and people do not pardon more than a sixth. If the victim wants the sale to stand, it stands. If the buyer overpaid and he waited longer than the time to show it, the seller cannot retract. We say that a seller can retract at any time when he was the victim. Perhaps the Rif agrees; we need not say that he allows the swindler to retract only when the victim claims the Ona'ah. If the price rose, some say that the buyer can claim the Ona'ah and validate the sale, for before the price rose, the seller needed to return the Ona'ah. R. Yonah says that he cannot, for Ona'ah is only at the time the sale is confirmed. Until now it was not, for he could have retracted, and now there is no Ona'ah. We do not say that once the price rose, surely he will not retract and the sale stands. This is why Rav Chisda said 'had you not overcharged me...'; he did not just say that retroactively the sale stands from the beginning and the price rose in the buyer's Reshus.

Ramban (83b DH Omar): A Ga'on proved from the Yerushalmi that Rav Chisda discusses Mi she'Pora. This is wrong. Our Mishnah discusses after Meshichah, and teaches that only the victim can void the sale. In Rav Chisda's case, the Mishnah would say that neither can retract, just like they could not retract after Meshichah. Rav Chisda holds that only the victim can retract. What is his source? Perhaps both may retract, for one need not fulfill his word for a sale with Ona'ah! Also, the Gemara should have said that this is Rav Chisda's Chidush, and not "Ra Ra Yomar..."? Also, If Rav Chisda discussed when only money was given, he would have specifed. The Rashbam and R. Chananel explain that he teaches that even if the price changed later, only the victim can retract. This is difficult. Surely this Chidush is not in the Mishnah. Also, change of price after Meshichah is meaningless! Really, Rav Chisda teaches that only the victim can retract. It does not depend on change of price, just normally, the swindler would not want to retract otherwise. If the Ona'ah is more than a sixth, the swindler can retract if the victim claims the Ona'ah. If the Ona'ah is a sixth, the swindler not retract in any case. There is no difference between retracting after Meshichah, after retracting without Mi she'Pora after giving money.

Rambam (Hilchos Mechirah 12:13): If Levi sold to David an item worth four for five, the sale is Batel. If before there was time to show it, the price rose to seven, David can retract but Levi cannot. David can say 'had you not overcharged me... Now that you overcharged me, can you retract?!'

Rambam (14): Similarly, if Levi sold an item worth five for four, and the price dropped to three, only Levi can retract. He can say 'because you underpaid me, will you retract?!'

Rambam (15): If Levi sold an item worth five for six, and before there was time to show it, the price rose to eight, Levi must return the overcharge (one), for David acquired and the item rose in price in David's Reshus. Similarly, if he sold an item worth six for five, and the price dropped to three, David must return one, for he acquired and the item declined in price in his Reshus.

Poskim

Shulchan Aruch (CM 227:10): If Levi sold to David an item worth four for five, the sale is Batel. If before there was time to show it to, the price rose to seven, David can retract but Levi cannot. David can say 'had you not overcharged me... Now that you overcharged me, can you retract?!'

SMA (21): The Rema (Sa'if 4) brought an opinion that the swindler can retract. Here all agree that he cannot, for once the price rose, presumably the buyer will not retract. If the buyer wants, he can retract until the time to show it.

Shulchan Aruch (11): Similarly, if Levi sold an item worth five for four, and the price dropped to three, only Levi can retract. He say 'you underpaid me; this does not entitle you to retract.'

Shulchan Aruch (12): If Levi sold an item worth five for six, and was unable to show it before the price rose to eight, Levi must return the one that he overcharged, for David acquired and the item rose in price in David's Reshus.

Beis Yosef (DH v'Heicha): Even though the Rosh connotes that he favors R. Yonah, who says that he cannot claim the Ona'ah, the Tur rules like the other opinion because he prefers its reasoning.

Gra (21): The Ramban explains that Rav Chisda teaches that the victim can claim the Ona'ah or void the sale. If so, the Halachah follows Rav Chisda.

Shulchan Aruch (13): Similarly, if he sold an item worth six for five, and the price dropped to three, David must return the one that he underpaid, for he acquired and the item declined in price in David's Reshus.

Bach (CM 227:7 DH Garsinan): To answer the contradiction between Rava and Rav Chisda, the Ga'on said that Rav Chisda discusses when money was given but there was no Meshichah. The victim of Ona'ah can retract without a Mi she'Pora. This is the Halachah. The Rif disagreed only about Bitul Mekach. He holds that either can retract only if the victim claimed the Ona'ah. If not, the swindler cannot retract. No one argues about Ona'ah of a sixth; only the victim can retract without Mi she'Pora.

Rebuttal (Shach 204:2): The Bach misunderstood the Rif. The Rambam explained well, and rejected the Ga'on. Indeed, the Nimukei Yosef connotes a bit like the Bach, but there is no proof from his words.