Mishnayos Bava Metzia Perek 4 Mishnah 3

The measure of exploitation for which one can claim that he was exploited is four silver ma’a from the twenty-four silver ma’a in a sela, or one-sixth of the transaction. Until when is it permitted for the buyer to return the item? He may return it only until a period of time has passed that would allow him to show the merchandise to a merchant or to his relative who is more familiar with the market price of merchandise. If more time has elapsed he can no longer return the item, as the assumption is that he waived his right to receive the sum of the disparity. The mishna continues: Rabbi Tarfon ruled in Lod: Exploitation is a measure of eight silver ma’a from the twenty-four silver ma’a of a sela, one-third of the transaction. And the merchants of Lod rejoiced, as this ruling allowed them a greater profit margin and rendered the nullification of a transaction less likely. Rabbi Tarfon said to them: Throughout the entire day it is permitted to renege on the transaction and not merely for the period of time it takes to show the purchase item to a merchant or a relative. The merchants of Lod said to him: Let Rabbi Tarfon leave us as we were, with the previous ruling, and they reverted to following the statement of the Rabbis in the mishna with regard to both rulings.

הָאוֹנָאָה, אַרְבָּעָה כֶסֶף מֵעֶשְׂרִים וְאַרְבָּעָה כֶסֶף לַסֶּלַע, שְׁתוּת לַמִּקָּח. עַד מָתַי מֻתָּר לְהַחֲזִיר. עַד כְּדֵי שֶׁיַּרְאֶה לְתַגָּר אוֹ לִקְרוֹבוֹ. הוֹרָה רַבִּי טַרְפוֹן בְּלוֹד, הָאוֹנָאָה שְׁמֹנָה כֶסֶף לַסֶּלַע, שְׁלִישׁ לַמִּקָּח, וְשָׂמְחוּ תַגָּרֵי לוֹד. אָמַר לָהֶם, כָּל הַיּוֹם מֻתָּר לְהַחֲזִיר. אָמְרוּ לוֹ, יַנִּיחַ לָנוּ רַבִּי טַרְפוֹן בִּמְקוֹמֵנוּ, וְחָזְרוּ לְדִבְרֵי חֲכָמִים: