29 Bava Metzia 4/3-6

(3) Fraud is [at least] four silver from twenty four silver [which constitute a] sela, [equaling] one-sixth of the sale price. Until when is it permitted to retract? Up to the time it takes to show it to a merchant or to one's relative. Rabbi Tarfon taught in Lod: Fraud is eight silver per sela, [which is] one-third of the sale price, and the merchants of Lod were happy. He said to them, "It is permitted to retract the entire day." They said to him, "Let Rabbi Tarfon leave us in our place," and they resorted to the words of the Sages.

(4) Both the purchaser and the seller have [the capacity to claim] fraud. Just as fraud applies to an ordinary person, so too fraud applies to a merchant. R. Yehudah says: Fraud does not apply to a merchant. The one who was defrauded - he has the upper hand. If he wants, he may say to him, "Give me my money," or "Give me the amount you defrauded me."

(5) How much can the sela lack [in weight] and it not be considered fraud? R. Meir says: Four issarin [at] one issar per dinar. R. Yehudah says: Four pundionot, [at] one pundion per dinar. R. Shimon says: Eight pundionot, [at] two pundionot per dinar.

(6) Until when is it permitted to retract [when the coin is lacking weight]? In cities, until it can be shown to a money-changer. And in villages: until the eves of the Sabbaths. If [the one who originally gave the coin] recognizes [the underweight coin], even after twelve months he should accept it from him. But [if he refuses to do so, the one who received the coin] only has a [legally unenforceable] complaint against him. And one may use it for ma'aser sheni without concern, for [one who refuses to receive it] is nothing but a bad person.