Reference for Bava Metzia 100:4
דאי סלקא דעתך דבטול מקח לרבנן בכדי שיראה לתגר או לקרובו במאי שמחו שמחו בשתות עצמה דלרבי טרפון מחילה ולרבנן אונאה
is only within the time that he can shew it to a merchant or to a relative, whereat did they rejoice?<span class="x" onmousemove="('comment',' The problem of the time within which the sale may be annulled is raised immediately after this passage. Here the Talmud anticipates it by pointing out that since the dealers originally rejoiced at R. Tarfon's ruling, which, ex hypathesi, means that from a sixth up to a third constitutes overreaching, it must be assumed that annulment in the view of the Rabbis is not limited by time. For otherwise, there was no reason to rejoice in the first place. The argument is this: There is very little practical difference between a whole day and always, because a day is quite ample for finding out that one was overreached; but there is a great difference between a day and the short time necessary for shewing one's purchase to a merchant, which may easily pass before the defrauded party discovers his loss. Furthermore, it is rare to overreach by more than a sixth (presumably buyers were very keen in those days!). Consequently, when R. Tarfon told them that a returnable overcharge is up to a third, which, as they thought, meant within the shorter period only, after which there was no redress, whilst in the view of the Rabbis the purchase could be annulled at any time if the overcharge was more than a sixth, R. Tarfon's ruling was naturally to their advantage. But if the annulment of the sale according to the Rabbis is only within the shorter period, why did they rejoice? On the contrary. R. Tarfon's ruling that up to a third constitutes overreaching as against the Rabbis' view that over a sixth annuls the sale was manifestly to their disadvantage: since according to the Rabbis both parties could withdraw, whilst on the view of R. Tarfon only the defrauded party had that right. ');"><sup>4</sup></span>