יורה דעה קס״הYoreh De'ah 165

Se’if 1א׳

הַמַּלְוֶה אֶת חֲבֵרוֹ עַל הַמַּטְבֵּעַ וְהוֹסִיפוּ עַל מִשְׁקָלוֹ, אִם הוּזְלוּ הַפֵּרוֹת מֵחֲמַת הַתּוֹסֶפֶת, מְנַכֶּה לוֹ שִׁעוּר הַתּוֹסֶפֶת, וַאֲפִלּוּ הוֹסִיפוּ עָלָיו כָּל שֶׁהוּא. וְאִם לֹא הוּזְלוּ מֵחֲמַת הַתּוֹסֶפֶת, אֵינוֹ מְנַכֶּה לוֹ אֶלָּא נוֹתֵן לוֹ מִמַּטְבֵּעַ הַיּוֹצֵא בְּאוֹתָהּ שָׁעָה. בַּמֶּה דְּבָרִים אֲמוּרִים, כְּשֶהוֹסִיפוּ עָלָיו עַד חֲמִישִׁיתוֹ, כְּגוֹן שֶׁהָיָה מִשְׁקָלוֹ ד' וַעֲשָׂאוּהוּ ה'. אֲבָל אִם הוֹסִיפוּ עָלָיו יוֹתֵר עַל חֲמִישִׁיתוֹ, מְנַכֶּה לוֹ כָּל הַתּוֹסֶפֶת, אַף עַל פִּי שֶׁלֹּא הוּזְלוּ הַפֵּרוֹת. וְהוּא הַדִּין לַמַּלְוֶה עַל הַמַּטְבֵּעַ וּפִחֲתוּ מִמֶּנּוּ. הַגָּה: וְעַיֵּן בח "ה סִימָן ע"ד אֵימַת יוּכַל לְהַחֲזִיר הַמַּטְבֵּעַ שֶׁהִלְוָה לוֹ, אַף עַל גַּב דְּנִפְסְלָה. וְאִם הוּזְלוּ הַפֵּרוֹת מֵחֲמַת הַמַּטְבֵּעַ הַחֲדָשָׁה, וְלֹא הוֹסִיפוּ עָלֶיהָ, נוֹתֵן לוֹ מַטְבֵּעַ הַחֲדָשָׁה (גְּמָרָא). וְהֵיכָא שֶׁעָשׂוּ מַטְבֵּעַ חֲדָשָׁה וְלֹא יָדַעְנוּ אִם הוֹסִיפוּ אִם פִּחֲתוּ חֹמֶשׁ, סָמְכִינָן אָאֻמָּנִים עוֹבְדֵי כּוֹכָבִים דִּבְקִיאִים בְּכָךְ, בִּמְסִיחִין לְפִי תֻּמָּן, אוֹ הָעַרְכָּאוֹת הַמְמֻנִּים עַל כָּךְ. (בֵּית יוֹסֵף בְּשֵׁם ס"ה). וְאִם גָּזַר הַמֶּלֶךְ שֶׁכָּל מִי שֶׁיִּפְרַע, יִפְרַע מִמַּטְבֵּעַ חֲדָשָׁה, הוֹלְכִים אַחַר גְּזֵרַת הַמֶּלֶךְ, דְּדִינָא דְּמַלְכוּתָא דִּינָא, וְאֵין בָּזֶה לֹא מִשּׁוּם רִבִּית וְלֹא מִשּׁוּם אִסּוּר גֶּזֶל (בֵּית יוֹסֵף בְּשֵׁם הָרַמְבַּ"ן ועי' ס"ק ח') ועי' בְּח"ה סִימָן ע"ד.

One who lends his fellow upon the coin, and they added to its weight: if the produce became cheaper on account of the addition, he deducts for him the measure of the addition, and that is so even if they added to it any amount whatever. And if it did not become cheaper on account of the addition, he does not deduct for him, but gives him of the coin that is current at that hour. In what case are these words said? Where they added to it up to a fifth of it, such as where its weight was four and they made it five. But if they added to it more than a fifth of it, he deducts for him the whole addition, even though the produce did not become cheaper. And the same law applies to one who lends upon the coin and they diminished from it. Hagah [Rema]: And see in Choshen Mishpat, siman 74, as to when he is able to return the coin that he lent him, even though it has been invalidated. And if the produce became cheaper on account of the new coin and they did not add to it, he gives him the new coin (Gemara). And where they made a new coin and we do not know whether they added or diminished a fifth, we rely upon craftsmen who are idolaters (oved kokhavim — the standing term for a gentile) and are expert in this, where they speak artlessly, or upon the courts that are appointed over this. (Beit Yosef in the name of Sefer haTerumot). And if the king decreed that whoever pays shall pay in the new coin, we follow the king's decree, for the law of the kingdom is law, and there is in this neither the matter of interest nor the matter of the prohibition of robbery (Beit Yosef in the name of the Ramban, and see se'if katan 8) and see in Choshen Mishpat, siman 74.