Gemara

(Mishnah): The Shi'ur (minimal quantity) for Ona'ah is four Kesef (silver coins, i.e. Ma'os, per Sela)... A Perutah is the Shi'ur for five things: the admission (for Modeh b'Miktzas)...

Question: Our Mishnah should also teach that the Shi'ur of Ona'ah is a Perutah!

Answer (Rav Kahana): The omission teaches that Ein Ona'ah l'Perutos (Ona'ah does not apply to Perutos).

(Levi): Ona'ah applies to Perutos.

(Levi's Tosefta): A Perutah is the Shi'ur for five things: Ona'ah... and a claim to obligate judges to judge the case.

Question: Why didn't our Tana teach that a Perutah is the Shi'ur of a claim to obligate judges to judge the case?

Answer: He already taught that it is the Shi'ur for theft.

56a (Mishnah): Ona'ah does not apply to documents.

56b (Beraisa): The verse forbidding Ona'ah says "v'Chi Simkeru." This connotes that the matter itself is sold. Documents are sold for the proof they contain (of a sale or loan). Therefore, if one sells documents to a perfumer (to use the parchment to cork his bottles), Ona'ah applies to them.

Objection: This is obvious!

Answer: This teaches unlike Rav Kahana, who says that Ein Ona'ah l'Perutos.

61a (Rava): Had the Torah not written a Lav of robbery, we could not have learned from Ona'ah, for a victim of Ona'ah does not know he was cheated, so he cannot pardon it. One who was robbed knows, and can pardon his loss.

Rishonim

Rif: The Halachah does [not - some texts omit this] follow Levi.

Rosh (4:20, and Chidushei Anshei Shem): The Rif says that the Halachah does not follow Levi, because the Mishnah connotes like Rav Kahana. I hold like the Ramah, who rules like Levi, like the Gemara on 56b. Levi explains that the Mishnah did not mention Ona'ah, for it is included in theft. The Gemara did not ask about this, rather, about which cases Beis Din judges, for that is according to everyone. The answer also answers for Ona'ah. If not, we would have challenged Levi, and answered that he holds like the Tana on 56b!

Rambam (Hilchos Mechirah 12:4): A swindler need not return Ona'ah unless it is more than a Perutah. If it was exactly a Perutah, he does not return it, for Ona'ah does not apply to Perutos.

Magid Mishneh: He rules that above a Perutah is Ona'ah, like Rav Kahana and the Stam Mishnah.

Kiryat Sefer: Even though one must return theft of a Perutah, regarding a sale, one pardons it, since it is not more than a Perutah. Likewise, if the buyer delayed, we say that he pardoned the Ona'ah.

Rosh (4:20): Even less than a sixth, Ona'ah is forbidden if it is at least a Perutah. "Al Tonu" forbids an amount considered to be money. Chachamim said that until a sixth is pardoned, because people cannot set prices exactly. Rav Kahana taught that Ona'ah does not apply to Perutos. Rashi explains that if the sale item is worth only six Perutos there is no Ona'ah, for this is less than an Isur, a (the smallest) silver coin (it is worth eight Perutos). Rashi erred. The smallest silver coin is a Mo'oh (32 Perutos). The Mishnah taught that the Shi'ur for Ona'ah is four Kesef out of 24 (a Sela). It did not specify (four of) which coin, for it was known that the smallest silver coin is a Mo'oh. The claim for Modeh b'Miktzas must be at least two Ma'os (Shevuos 39b). We learn from "Ki Yiten... Kesef O Kelim." Just like Kelim (plural) implies two, also Kesef. If there were a smaller silver coin, it should be two of that coin! Also, a Tosefta says that copper acquires silver, e.g. if one gave 30 Isarim for a Dinar Kesef... This shows that Isarim are of copper! The Rif brought this Tosefta at the beginning of the Perek. Once we exclude Perutos from Ona'ah, the Shi'ur is a Mo'oh.

Nimukei Yosef: The Rif rules like Levi. Also the Rambam does, for he says that one must return anything above a Perutah.

Rashash (55a DH Ein): On 56b, Rashi says that Ein Ona'ah l'Perutos means that he paid less than an Isar, so there is no Ona'ah until he paid an Isar and the Ona'ah is a one and a third Perutos. The Rosh explains that Rashi requires the Ona'ah to be at least an Isar; the Rosh himself requires at least a Mo'oh. The Rambam holds that Ona'ah can be anything above a Perutah. Why does the Nimukei Yosef say that the Rambam rules like Levi? Perhaps he means that the Rambam does not hold like Rashi, who requires an Isar. It seems that Rav Kahana does not hold that Ona'ah applies to anything above a Perutah, for we say that it does not apply to documents sold for the paper. The Ona'ah on them could be more than a Perutah!

Machaneh Efrayim (Ona'ah 17): The Rosh says that a Lav forbids Ona'ah less than a sixth. If so, there should be lashes for this, but we never heard of lashes for Ona'ah.

Minchas Chinuch (337): Surely, one transgresses even for Ona'ah of a Perutah, just he need not return it. Since he does not pay, he should be lashed! The opinion that Ein Ona'ah l'Perutos is because he pardons. Why is this different than all other monetary laws, for which the Shi'ur is a Perutah? We must say that Chachamim knew that in sales, one pardons a Perutah. We can say that he is not lashed, like I explained regarding Ona'ah less than a sixth (it is considered that he is liable to pay, even though he does not pay).

Poskim

Shulchan Aruch (CM 227:5): A swindler need not return Ona'ah unless it is more than a Perutah. If it was exactly a Perutah, he does not return it.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Tur says that the Rif says that once we exclude Perutos from Ona'ah, the Shi'ur is a Mo'oh. The Rosh explained the Rif this way; we could explain differently. Perhaps the Rif holds like the Rambam, that Rav Kahana excludes only a Perutah, but more than a Perutah is Ona'ah!

Drishah (1 DH u'Bilvad): The Tur says that the Rif requires Ona'ah to be at least a Mo'oh. Rashi explicitly says that the Shi'ur is an Isar, and the Rambam holds that more than a Perutah is Ona'ah. Rashi admits that it must be a silver coin. The Rosh proved from a Tosefta that an Isar is of copper; he mentioned that the Rif brought this Tosefta, to teach that also the Rif holds that the Shi'ur is a Mo'oh. The Tur holds that also the Rambam (Hagahah under Rif in Oz v'Hadar Gemara - this should say 'Ramah') means that once we exclude Perutos from Ona'ah, the Shi'ur is a Mo'oh. An Isar is not different than a Perutah. The Rif and Rambam mentioned a Perutah only because the Mishnah and Amora'im did.

Gra: Rashi says that Ona'ah applies only to at least an Isur, which is of silver. The Amora'im argue about whether or not there is Ona'ah for Perutos; this supports Rashi. Also in Kidushin (2a), Rashi said that the Isar is of silver. This is wrong.

Piplulei Charifta (4:20): In Shevuos 39b, Rashi explained like the Rosh that a Mo'oh is the smallest silver coin.

Note: The Sugya is also brought in Kidushin 11b, and there Rashi explained that a Mo'oh is the smallest silver coin in Tzuri.

Rema: Some say that he must return a Perutah.

Gra (11 and 6:1): The first opinion (the Rambam) rules like Rav Kahana, for the Stam Mishnah is like him. How could Levi know better than Rebbi?! The latter opinion holds that the Gemara did not need to answer how Levi explains the Stam Mishnah.

Drush v'Chidush (R. Akiva Eiger 61a DH v'Im): Rava said that we could not learn robbery from Ona'ah, for a victim of Ona'ah does not know he was cheated. Rava must hold like Levi. According to Rav Kahana, in any case we cannot learn, for robbery applies even to a Perutah!

Note: Rava was Basra (later than Rav Kahana and Levi). I did not notice anyone saying that this is a reason to rule like Levi!