Maaser Sheni, Chapter Three, Mishnah Five
In our mishnah we learn that all of the sages agree that maaser sheni money can be brought out of Jerusalem once it has been brought in. However, there is a debate whether second tithe produce can be taken out of Jerusalem once it has already been brought in. According to the first opinion, once it has been brought into Jerusalem it can no longer be brought out. The point of the laws of maaser sheni is for the produce to be eaten in Jerusalemtherefore once it has been brought into Jerusalem, it can no longer be brought out. In contrast, maaser sheni money is just a means through which to transfer the sanctity of some maasser sheni produce to other maaser sheni produce. Hence, one can bring it out of Jerusalem.
Rabban Shimon ben Gamaliel holds that even produce can be brought in and taken out of Jerusalem.
1) [Second tithe] money may be brought into Jerusalem and taken out again, but [second tithe] produce may only be brought in, but not taken out again.
2) Rabban Shimon ben Gamaliel says: produce also may be brought in and be taken out again.