Maaser Sheni, Chapter Two, Mishnah Six
This mishnah deals with a case of a sela of maaser sheni and a sela of hullin that gets mixed up and he wants to take the hullin sela out of Jerusalem.
1) A sela of maaser sheni which was mixed up with a sela of hullin, one may bring copper coins for the sela and say: Let the sela of maaser sheni wherever it may be, be exchanged for these coins.
2) Then he must select the better of the two selas, and change [again] the copper coins for it.
3) For they have said: one may change silver for copper [only] in case of necessity, and not to leave it so but to change it again for silver.
Section one: After the coins are mixed up, he can declare that one of the coins is exchanged for an equivalent value of copper coins.
Section two: Then he takes the better of the two selas and redeems the copper coins for this sela. In this way he has transferred maaser sheni from one of the two selas to the copper coins and then back to the better sela. The better sela ends up being maaser sheni and the copper coins and the worse sela are hullin.
Section three: The mishnah explains why he cant simply exchange the copper coins for one of the selas and be done with it. Also, why cant he exchange one silver sela for another? Why does he have to bring copper coins to exchange for the silver sela? The mishnah teaches that changing silver to copper is allowed, but only if there is no other way to solve the problem. By implication, changing silver coins for other silver coins is never allowed. And even when he is allowed to change copper for silver, he can only do so temporarilyas soon as he has done so he must change the copper back for silver.