Maasrot, Chapter Two, Mishnah Four
Our mishnah deals with a person who separated terumah from his produce before he finished its processing and before he separated the tithes.
1) Produce from which he separated terumah before its work was finished:
a) Rabbi Eliezer says: it is forbidden to make a chance meal of it,
b) But the sages permit it except when it is a basket of figs.
2) A basket of figs from which one separated terumah:
a) Rabbi Shimon permits it.
b) But the sages forbid it.
Section one: According to Rabbi Eliezer, once one has separated terumah from produce it can no longer be eaten in a chance fashion until tithes have also been separated. To put it another way, taking out terumah makes the produce liable for tithes.
The rabbis generally disagree and hold that separating terumah does not make produce liable for tithes. One can continue to eat chance meals from the produce. The one exception is a basket of figs.
Albeck tentatively explains that it was common to give figs to several people (as we saw above in mishnayot 1-2) and if he took out terumah then he has shown that his intention is to give away the figs while they are in the basket and therefore this is considered the final step in their processing. Furthermore, once he has separated the terumah he wont put more hullin figs into the basket and therefore, their processing is complete. Therefore, he can no longer eat in a chance fashion from these figs.
Section two: Rabbi Shimon does not distinguish between figs and other types of produceeven though he separated terumah from the basket, it is still not liable for tithes and he can continue to eat in a chance fashion.