Maaser Sheni 2:6-7

Ma’aser Sheini 2:6

If a sela (a denomination of money) of second tithe gets mixed up with a sela of secular money, one must bring a sela’s worth of coins and say, “A sela of tithe, wherever it is, is redeemed with these coins.” He takes the better sela and uses that to redeem it because the Sages said that silver may be redeemed with copper if necessary but the copper may not be left sanctified – it must be redeemed back onto silver.

Ma’aser Sheini 2:7

Beis Shammai say that one may not redeem silver coins onto gold (in order to carry fewer coins to Jerusalem) but Beis Hillel permit it. Rabbi Akiva reported that he personally exchanged second tithe money belonging to Rabban Gamliel and Rabbi Yehoshua for gold coins.