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If a seller is a victim of Ona'ah he has the option of demanding the purchase item back and retract from the sale or he may maintain the sale and demands the extra sixth that he undercharged by.
It is a Safek if a person is Mochel immediately if he is over or undercharged less than a sixth or he is only Mochel after he has sufficient time to show the item to a merchant or the seller. (1)
The Chachamim hold if someone is under or overcharged by a sixth it is Ona'ah, while R. Tarfon holds that if someone is under or overcharged by a third it is Ona'ah. (2)
The Chachamim hold that a buyer who is the victim of Ona'ah may only claim the Ona'ah within the time it takes to show the item to a merchant or a relative.
R. Tarfon holds that a victim of Ona'ah may claim the Ona'ah for the duration of the day.
It is a Safek if someone is overcharged by more than a sixth may retract the sale forever or only within the time to show the item to a merchant or a relative. (3)
R. Nasan says if the seller undercharged by a sixth the sale is valid and the Ona'ah must be returned, while Rebbe holds that the seller has the option to retract from the sale or to demand his Ona'ah back.
R. Nachman says that a buyer who was overcharged may only claim the Ona'ah within the time it takes to show it to a merchant or a relative but a seller who undercharged may claim the Ona'ah forever. (4)
A BIT MORE
1. If he is only Mochel after he has a chance to show it to a merchant or a relative the difference between less than a sixth and a sixth is that a person who is overcharged by a sixth may retract from the sale completely or he may demand that the extra sixth be returned while someone who was overcharged by less than a sixth may not retract from the sale and may only demand the extra U
2. R. Tarfon holds that from one sixth until one third it is Ona'ah while the Chachamim hold that one sixth is Ona'ah and more than a sixth is Bitul Mekach.
3. If he may only retract within the time that it takes to show the item to a merchant or a relative the different between a sixth and more than a sixth is that if someone was over or undercharged by a sixth only the victim may retract but not the perpetrator but if someone is overcharged by more than a sixth both the perpetrator and the victim may retract.
4. The buyer has the purchase item in his hand so he can show it off and find out if he got a good deal, however the seller doesn't have anything to show off and he will only find out that he undercharged when he sees a similar item selling for more than he sold it for.
Rava says the Halachah is that if a person is under or overcharged by exactly a sixth he may demand that the Ona'ah be returned to him, however he may not retract from the purchase. The Ramban says if a person was overcharged a sixth and he gave the money but no Kinyan was made if he retracts he is cursed with a Mi She'Para. Even though he was overcharged since the Halachah is that he only has the right to demand the Ona'ah but not to retract, if he decides to retract even though no Kinyan was made he is cursed with a Mi She'Para.
CHIYUV FOR ONA'AH
It is forbidden to gyp you friend and if either a buyer or a seller gyps his friend it is a transgression of a Mitzvas Lo Sa'aseh. How much does the Ona'ah have to be to obligate him to return it? Exactly one sixth. What is the case? If he sold something worth six for five or something worth seven for six or something worth five for six or something worth six for seven it is Ona'ah and the purchase is valid and the perpetrator must pay the entire amount of the Ona'ah to the victim. If the Ona'ah is less than a sixth by even a slight amount for example he sold something worth seventy for sixty plus a Perutah he is not Chayav to return anything because it is usual for all people to be Mochel less than a sixth. (Shulchan Aruch CM 227:1, 2, 3)
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