Bava Metzia 50B

ולרבי טרפון נמי לאלתר הויא מחילה, אמאי חזרו? בדרבי טרפון ניחא להו טפי, דמאי דרבנן קא משוי להו אונאה לרבי טרפון הויא מחילה?

and according to R' Tarfon too, there is a waiver of the disparity of less than 1/3rd and the transaction is finalized immediately, why did they revert to following the statement of the Rabbis? In that case, the ruling of R' Tarfon would be preferable for them, as that which the Rabbis deem exploitation, i.e., a discrepancy of 1/6th, is waived according to R' Tarfon.

מי סברת פחות משליש לרבי טרפון כפחות משתות לרבנן דמי? לא, משתות ועד שליש לרבי טרפון כשתות עצמה לרבנן דמי. אי הכי, במאי שמחו מעיקרא?

The Talmud rejects this proof: Do you maintain that the legal status of a disparity of less than 1/3rd according to the opinion of R' Tarfon is like the legal status of a disparity of less than 1/6th according to the opinion of the Rabbis? No, the legal status of a disparity ranging from 1/6th until 1/3rd according to the opinion of R' Tarfon is like the legal status of a disparity of 1/6th itself according to the opinion of the Rabbis, and the exploited party receives the sum of the exploitation in return. The Talmud asks: If so, for what reason did the merchants of Lod rejoice initially? They gained nothing relative to the ruling of the Rabbis.

תפשוט דבטול מקח לרבנן לעולם חוזר, דכיון דאמר להו רבי טרפון הויא אונאה ⁠– שמחו, כי אמר להו כל היום ⁠– חזרו.

Resolve, based on this difficulty, the dilemma raised below, and conclude that in cases of nullification of the transaction according to the Rabbis, one may always renege on the transaction. Therefore, the reaction of the merchants of Lod is understandable, as, since R' Tarfon said to them that a disparity between 1/6th and 1/3rd is merely exploitation, they rejoiced, as this would mean that the buyer has only the time it takes to show the merchandise to a merchant or a relative to renege. When he said to them that the exploited person can renege on the transaction for the entire day, they reverted to following the statement of the Rabbis.