Bava Metzia 52B

יתר על כן מוכרה בשויה. מאי לאו: שפחתה יותר מכדי אונאתה? לא, יתירה: דאכתי לא פחתה בכדי אונאתה, מוכרה בשויה.

If the erosion was greater than that, he may sell the coin at its value as metal, not for its original value. What, is it not that it eroded more than its measure of exploitation? The Talmud rejects this claim: No, the term greater means that if it did not yet erode to its measure of exploitation, he may sell it at its value.

מתיבי: עד כמה תיפחת, ויהא רשאי לקיימה? בסלע ⁠– עד שקל. מאי לאו דפחית פורתא פורתא! לא, דנפיל לנורא ואפחית בחדא זימנא.

Another objection was raised: To what extent can the coin erode and one would still be permitted to maintain it? For a sela that deteriorated, it is permitted to maintain it up to a shekel, which is half its value. Does this not mean that the sela depreciated little by little, which indicates that even though it lost half its value, which is far greater than the measure of exploitation, it is still permitted for one to maintain it, and there is no concern about deceit? The Talmud rejects this: No, it is referring to a coin that fell into the fire and eroded all at once, and therefore no deceit is possible.

אמר מר: יקבנה ויתלנה בצואר בנו או בצואר בתו. ורמינהו: לא יעשנה משקל בין משקלותיו, ולא יזרקנה בין גרוטותיו, ולא יקבנה ויתלנה בצואר בנו ובצואר בתו. אלא: או ישחוק, או יתוך, או יקוץ, או יוליך לים המלח!

The Master said in the baraita: If a coin greatly depreciated he should perforate it, and suspend it as an ornament on the neck of his son or the neck of his daughter. And the Talmud raises a contradiction from a baraita: With regard to an eroded coin, one should not make it a weight among his weights, nor cast it among his metal scraps [gerutotav], nor perforate it and suspend it on the neck of his son or the neck of his daughter, lest he come to use it by mistake. Rather, he should either grind it or melt it, or cut it into pieces, or take it and cast it into the Dead Sea.