The [amount that constitutes] fraud is four silver [ma’os] out of twenty-four silver [ma’os which are equivalent] to a sela —- a sixth of the purchase.  Until when can one revoke [the sale]? Until he is able to show [the item] to a merchant or to his relative.  R’ Tarfon ruled in Lod: The [amount that constitutes] fraud is eight silver [ma’os] out of a sela —- a third of the purchase —- and the merchants of Lod rejoiced. He said to them: For a whole day, one may revoke [the sale]. They said to him: Let R’ Tarfon leave us alone in our place. And they went back to the words of the Sages.

Both the buyer and the seller are governed by the [law of] fraud. Just as [the law of] fraud [applies] to a layman, so does [the law of] fraud [apply] to a merchant. R’ Yehudah says: [The law of] fraud does not [apply] to a merchant.  The one who has been imposed upon has the upper hand. If he wishes, he may say to him: “Give me my money”, or: ”Give me what you defrauded me.”