Talmud for Maaser Sheni 4:2
פּוֹדִין מַעֲשֵׂר שֵׁנִי בְּשַׁעַר הַזּוֹל, כְּמוֹת שֶׁהַחֶנְוָנִי לוֹקֵחַ, לֹא כְמוֹת שֶׁהוּא מוֹכֵר. כְּמוֹת שֶׁהַשֻּׁלְחָנִי פוֹרֵט, וְלֹא כְמוֹת שֶׁהוּא מְצָרֵף. וְאֵין פּוֹדִין מַעֲשֵׂר שֵׁנִי אַכְסָרָה. אֶת שֶׁדָּמָיו יְדוּעִים, יִפָּדֶה עַל פִּי עֵד אֶחָד. וְאֶת שֶׁאֵין דָּמָיו יְדוּעִים, יִפָּדֶה עַל פִּי שְׁלֹשָׁה, כְּגוֹן הַיַּיִן שֶׁקָּרַם וּפֵרוֹת שֶׁהִרְקִיבוּ וּמָעוֹת שֶׁהֶחֱלִיאוּ:
One may redeem <i>Ma'aser Sheni</i> according to its cheapest value, in the manner that the storekeeper pays, not in the manner that he sells. In the manner that the money changer splits [coins], not in the manner that he puts [them] together. One may not redeem <i>Ma'aser Sheni</i> by estimate. If its cost is known it may be redeemed based on one (witness), and if its cost is not known it may be redeemed based on three [witnesses], for example wine that has formed a skin, fruit that spoiled, and coins that have become rusty.
Jerusalem Talmud Maaser Sheni
One redeems Second Tithe following the low rate at which the grocer buys39At wholesale., not at which he sells; the one at which the banker gives small coins, not at which he assembles large coins40The banker engaged in exchange operations makes money on the spread between buying and selling rates. For redeeming tithe one values bronze coins as if one were a banker selling copper coins against silver.. One does not redeem Second Tithe en bloc41Cf. Demay Chapter 2, Note 198.; if the price is known it is redeemed by the information of one person, if the price is not known by the information of three, as e. g., wine getting sour, fruit rotting, or coins rusting.