Talmud for Maaser Sheni 2:6
סֶלַע שֶׁל מַעֲשֵׂר שֵׁנִי וְשֶׁל חֻלִּין שֶׁנִּתְעָרְבוּ, מֵבִיא בְסֶלַע מָעוֹת וְאוֹמֵר, סֶלַע שֶׁל מַעֲשֵׂר שֵׁנִי, בְּכָל מָקוֹם שֶׁהִיא, מְחֻלֶּלֶת עַל הַמָּעוֹת הָאֵלּוּ, וּבוֹרֵר אֶת הַיָּפָה שֶׁבָּהּ, וּמְחַלְּלָן עָלֶיהָ, מִפְּנֵי שֶׁאָמְרוּ, מְחַלְּלִין כֶּסֶף עַל נְחֹשֶׁת מִדֹּחַק, וְלֹא שֶׁיִּתְקַיֵּם כֵּן, אֶלָּא חוֹזֵר וּמְחַלְּלָם עַל הַכָּסֶף:
If a <i>Selah</i> [specific unit of weight] of <i>Ma'aser Sheni</i> and of <i>Chulin</i> that were mixed together, he brings in a <i>Selah</i> of money and says, "The <i>Selah</i> of Ma'aser Sheni, wherever it is, becomes <i>Chulin</i> [by transferring its holiness] onto this money," and sorts the best of them, and makes them <i>Chulin</i> on it, for they said, one may make silver <i>Chulin</i> [by transferring its holiness] onto copper in an exigency, and not so it will remain so, but rather one must return and make them <i>Chulin</i> [by transferring their holiness] onto silver.
Jerusalem Talmud Maaser Sheni
If a tetradrachma of Second Tithe was mixed up with a profane one91And it is not known which of the two coins is dedicated as Second Tithe. The farmer does not want to take both coins to Jerusalem., one brings coins in the value of a tetradrachma and says: The tetradrachma of Second Tithe, wherever it may be, is exchanged for these coins. Then he chooses the better of the two and exchanges it for them since they said, one exchanges bronze for silver in an emergency92But not silver for silver. When the silver coin, the standard legal tender in the entire Roman empire, is spent in Jerusalem it is usually first converted into local bronze coin. Therefore, this exchange is legal under certain circumstances. But silver coins under normal circumstances are never exchanged into other silver coins and, therefore, such an exchange is never permitted. The bronze coins should not be kept as such because non-local bronze coins may not be accepted at full value in Jerusalem., not that it should stay so but that he should exchange them for silver.
The House of Shammai say, a person should not turn his tetradrachmas into gold denars but the House of Hillel permit it. Rebbi Aqiba said, for Rabban Gamliel and Rebbi Joshua I turned their silver coins into gold denars.