Reference for Bava Batra 189:5

הכי השתא התם מעיקרא שוה בשוה א"ל מיהו פחות משתות לא ידיעה במנה ומחיל איניש שתות ידיעה ולא מחיל איניש יתר משתות מקח טעות הוא ובטל מקח

Come and hear! [We learned]: [If] the overcharge is less than a sixth, the purchase is valid;<span class="x" onmousemove="('comment',' Lit., 'possession is acquired'. and nothing of the overcharge need be returned. Any buyer is assumed to be indifferent to the loss of such a small amount as a sixth. ');">⁸</span> [if it is] more than a sixth, the purchase is cancelled; [if it is] a sixth, the sale is valid but the overcharge must be refunded.<span class="x" onmousemove="('comment',' B.M. 50b. ');">⁹</span>

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