TT #771 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:13
You shall not have in your bag two kinds of weights, a large and a small.
Torah Temimah points to Sifrei / Bava Metzia.
TT lemma: לֹא יִהְיֶה לְךָ בְּכִיסְךָ.
תַּנְיָא רַבִּי עֲקִיבָא אוֹמֵר: מִנַּיִן שֶׁלֹּא יְקַיֵּם סֶלַע פָּחוֹת מִשֶּׁקֶל וְלֹא דִּינָר פָּחוֹת מִטַּרְפְּעִיק? תַּלְמוּד לוֹמַר לֹא יִהְיֶה לְךָ.
English: It was taught: Rabbi Akiva says, From where do we know that one may not keep a sela that has become less than a shekel, nor a dinar that has become less than a tarpeik? Scripture says “you shall not have.”
On the daf / in context: Worn or underweight coins that can still be passed as full weight are forbidden to retain.
Derash prohibiting the retention of defective weights/coins.
One may not keep underweight coins that could be used to defraud.