TT #779 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:15
A complete and just weight you shall have, a complete and just measure you shall have, that your days may be prolonged in the land that the Eternal your God is giving you.
Torah Temimah points to Bava Batra 89a.
TT lemma: אֵיפָה שְׁלֵמָה וָצֶדֶק.
וּמִנַּיִן שֶׁאִם אָמַר הֲרֵינִי מוֹחֵק בִּמְקוֹם שֶׁגּוֹדְשִׁין וּלְפָחוֹת לוֹ מִן הַדָּמִים, וְהֲרֵינִי גּוֹדֵשׁ בִּמְקוֹם שֶׁמּוֹחֲקִין וְאוֹסִיף לוֹ עַל הַדָּמִים, שֶׁאֵין שׁוֹמְעִין לוֹ? תַּלְמוּד לוֹמַר אֵיפָה שְׁלֵמָה וָצֶדֶק יִהְיֶה לָּךְ.
English: And from where that if one says “I will level off in a place where they heap and reduce the price for him, or I will heap in a place where they level off and increase the price for him,” that we do not listen to him? Scripture says “a complete and just ephah you shall have.”
On the daf / in context: Even with a compensating price adjustment, the deviation from the local measuring custom is not permitted.
Derash rejecting compensatory deviation from custom.
One may not deviate from the local measuring custom even if one adjusts the price accordingly.