Related for Bava Metzia 111:1
מפני שאמרו מחללין אותו כסף על נחשת מדוחק לא שיקיים כן אלא שחוזר ומחללן על הכסף קתני מיהת מחללין מדוחק מדוחק אין שלא מדוחק לא
because It was said, It [sc. the second tithe] may be redeemed [by substituting] copper for silver in case of emergency; not, however, that it should remain so, but that it should itself be redeemed in turn with silver.<span class="x" onmousemove="('comment',' M.Sh. II, 6. This states the reason of this cumbersome procedure. For one might have thought a much simpler procedure possible, viz., one of the sela's could be taken and the following declaration made: 'If this is the second tithe sela', it is well; but if not, let this redeem the other.' — Therefore the Mishnah states that even the substitution of copper coin for silver was permitted only in an emergency, but silver can in no circumstance be used for redeeming other silver, since it cannot be regarded as substitution when both are of the same metal. Nevertheless, it was not desirable that the second tithe should remain in the form of copper, because it was liable to corrosion, and moreover, silver was a more dignified and worthier form of exchange than copper. Therefore the copper coins had to be redeemed in turn with the best of the two sela's. ');"><sup>1</sup></span> Thus it is nevertheless stated that it [silver] may be exchanged in case of emergency, proving that only in an emergency is it done, but not otherwise!<span class="x" onmousemove="('comment',' Whereas in the case of demai it was stated on R. Meir's authority that even silver may be freely employed in redeeming silver and copper may redeem silver even without any emergency, thus proving that demai is treated more leniently than certain tithe. This contradicts R. Meir's previous statement that demai was enacted with the same stringency as certain tithe. Though, of course, a Mishnah cannot be employed to prove R. Meir wrong, since R. Meir, as a Tanna, could disagree, the point here is that this Mishnah is anonymous, and it is a Talmudic principle (Sanh. 86a) that an anonymous Mishnah agrees with R. Meir. — Rashi. For Tosaf.'s interpretation, which differs considerably from this, v. p. 331, nn. 2, 3. ');"><sup>2</sup></span>