Bava Metzia, Chapter 4, Mishnah 4. This Mishnah continues from where the previous Mishnah left off, and begins by stating: "Echad halokei'ach v'echad hamocheir yesh lahen ona'ah" - the laws of ona'ah apply both to the buyer and to the seller.
That is to say, in cases where the law applies, one may not charge too much and one may not pay too little: whether the person is the buyer who paid too much, or the seller who received too little in exchange for his merchandise; whether it was done knowingly, where he agreed to a bad deal, or whether it happened by mistake (such as omitting a digit when calculating the price).
Ona'ah for an ordinary person and ona'ah for a merchant:
"Keshem she'ona'ah lahedyot, kach ona'ah latagar" - the law applies whether the buyer or seller is an ordinary person, or whether he is a merchant: a dealer and professional, like a wholesaler who constantly engages in buying and selling and knows the prices well. Nevertheless, the Tanna Kamma states that if the merchant ultimately agreed to a bad deal, even though he knows the prices, he can retract his agreement and claim that there was deception and exploitation here, and the deal is unfair, and therefore he wishes to void it.
The opinion of Rabbi Yehudah:
"Ein ona'ah latagar" - Rabbi Yehudah disagrees. Since the merchant is an expert in his trade and knows the price of that merchandise well, he cannot claim that he did not know or that he made a mistake. His agreement to the price indicates that he knowingly waived his rights, and this is not ona'ah. The concept of ona'ah is about extortion and exploitation, and here the person was not exploited: with his eyes open, he knew what a fair price was, and presumably he needed the money for some reason and agreed to the deal, which is very lucrative for the other party. Therefore, he has no claim.
However, the opinion of the Tanna Kamma is that the law of ona'ah applies equally to the buyer and the seller, even when he is an expert in evaluating the worth of these items.
"Mi shehutal alav yado al ha'elyonah" - the person who was subjected to the deception, meaning the one who lost out and received the bad deal, has the upper hand, and he has the choice:
"Ten li ma'otai" - to retract from the deal and demand his money back.
"O ten li mah she'onitani" - to settle for the return of the difference that was overcharged or underpaid to him, in order to set the deal at a fair price.
The measure of ona'ah and the halachic ruling:
The Tanna of our Mishnah, Rabbi Yehudah HaNasi, holds that the law takes effect exactly at the measure of a sixth (shtut), and the one who was subjected to ona'ah has two options: to void the sale or to demand his money back - and this applies at exactly a sixth.
However, the halachah is not ruled according to our Mishnah, but rather according to the opinion of the Tanna Rabbi Natan, who was at times, in the early stages, a disputant of Rabbi Yehudah HaNasi, and was known as a judge with particular expertise in monetary laws. He delved into the depths of the halachah and determined, and so it is ruled:
An exact ona'ah of a sixth: One who paid too much or received too little receives the difference back, but he cannot cancel the sale and restore the situation to how it was.
An ona'ah of more than a sixth: As the Mishnah states, the choice is his - if he wishes, he can claim his money back, and if he wishes, he can cancel the transaction.
The asymmetry between the buyer and the seller:
The halakhic authorities, including the Rambam, ruled that even the seller can cancel the transaction if he suffered a loss, but there is an asymmetry here. The one holding the merchandise after the sale, who is the buyer, has only a limited window of time, as explained in the previous Mishnah: the amount of time required to show the item to an expert or his relative and return. In contrast, the seller, who no longer has the item in his possession, has no one to show it to, and the matter is not brought to his attention. Therefore, if after some time, three days for example, he realizes that the transaction was unfair and that he took a loss, he may request its cancellation - provided that the discrepancy was more than a sixth.
The seller's right versus the claim for the difference:
The Rambam adds, as many Rishonim learn, that the defrauded party can claim his money back or cancel the purchase entirely. In a scenario where he chose to claim his money back, many Rishonim learn that at that point, the merchant who sold him the goods can say: "I am not returning the money to you, rather I am canceling the transaction." In other words, the merchant may also choose to cancel the purchase and back out - provided, according to the majority of the Rishonim, that he cannot initiate the cancellation, but he is allowed to demand cancellation when the alternative facing him is returning the profit he earned in the transaction.
In summary: In this Mishnah we learned that the laws of ona'ah apply to the buyer and the seller alike, and according to the Tanna Kamma even to a merchant who is an expert in prices, whereas Rabbi Yehuda holds "ein ona'ah latagar" - a merchant is not subject to the laws of ona'ah, since his knowing agreement is considered a waiver. We discussed the right of the defrauded party - "ten li ma'otai" - give me my money, or "ten li mah she'onitani" - give me what you overcharged me - and the halakhic ruling in accordance with Rabbi Natan: with exactly a sixth, only the difference is returned, and with more than a sixth, the choice is his. Also explained was the asymmetry between the buyer, whose time is limited, and the seller who does not have the item in his possession, as well as the right of the merchant to demand cancellation of the sale as an alternative to returning the profit.