Beis Shammai say: A person may not change his sela coins into gold dinars. But Beis Hillel permit it. R' Akiva said: I made, for Rabban Gamliel and for R' Yehoshua, their silver into gold dinars.

[If] a sela of maaser sheni and of chullin became mixed together, he brings a sela's worth of coins, and says: “The sela of maaser sheni, wherever it is, is deconsecrated on these coins,” and he selects the nicer in it and redeems them on it, for they said: One may deconsecrate silver on copper in exigencies, but not to retain it in this way; rather, he deconsecrates them again on silver.