Maaser Sheni, Chapter Five, Mishnah Seven
This mishnah addresses the question of what to do with maaser sheni once the Temple has destroyed and it is impossible to eat it in Jerusalem.
One who had produce at this time and the time of removal arrived:
1) Bet Shammai says: he must exchange it for money.
2) But Bet Hillel says: it is all the same whether it becomes money or it remains fruit.
Section one: According to Bet Shammai, when the Temple no longer stands one should redeem maaser sheni produce with money and then put the money aside. Better to have the money sitting around then the produce. Perhaps Bet Shammai reasons that money will last longer than produce, so if he redeems the produce for money there is a chance that when the Temple is rebuilt he can bring it to Jerusalem.
Section two: Bet Hillel says it doesnt matter whether the maaser sheni is produce or money, it must be removed at the end of the third or sixth year, because it cant be brought to Jerusalem when the Temple no longer stands. So he should just leave the maaser sheni as produce and let it rot (see above 1:5).