Mishnah 4 in the second chapter of Maseches Terumos deals with separating Terumah between different species: from which species and from which quality a person may separate.
"Ein torimin mimin al she'eino mino" - one does not separate Terumah from one species for another species:
Terumah may not be separated from one type of food to cover a different type of food; rather, each and every species requires its own separation. How so? A person has before him a vessel of wheat and a vessel of barley, and each one contains fifty cups - he must separate one cup from the wheat and one cup from the barley. If he separated two cups of wheat to cover both, even though both are grain, he has not fulfilled his obligation.
"Ve'im taram, ein terumaso terumah" - the separation does not take effect at all, and he must go back and separate again properly.
"Kol min chittim echad" - all types of wheat are one species:
All varieties of wheat are considered a single species, and therefore one may separate from one for the other. Even though we are not familiar with wheat in all its many varieties, the principle applies equally to the produce we do have on hand: different varieties of rice, or different varieties of apples - all of them are one species in this regard.
"Kol min te'enim ugrogaros udveilah echad" - all types of figs, dried figs and pressed fig cakes are one species:
In the days of the Mishnah, figs reached the table in three distinct forms:
Te'enah - the fresh fruit.
Grogeres (plural: grogaros) - a dried fig, less tasty than the fresh one but lasting far longer.
Develah - figs pressed together into one mass, a sort of mashed fig cake. This is an intermediate form: tastier than a dried fig, and longer lasting than a fresh fig.
These three forms are considered one species. Therefore, if a person had fifty litras of each one of them, he may separate three litras from one of them to cover all of them.
From which of them is it proper to separate?
"Kol makom sheyesh kohen" - anywhere that a kohen is available - "torimin min hayafeh" - one separates from the finest, meaning from the choicest and tastiest: from the fresh figs.
"Kol makom she'ein kohen" - anywhere that a kohen is not available - "torimin min hamiskayem" - one separates from what will keep. When no kohen is around, and there is no certainty that he will reach the Terumah before it spoils, one separates from the type that will last over time: from the dried figs.
The Tanna Kamma, whose view is the accepted halachah, certainly agrees with the Torah's requirement of "chelbo" - that one give from the very finest; he simply holds that the focus should be on what the kohen will actually receive. Although at the moment of separating, the fresh fig is the better gift, by the time it reaches the kohen's hands - after some time has passed - the dried fig will be far superior, since the fresh one will have spoiled by then. Therefore one separates from whichever type will be the finest when it reaches the kohen.
The View of Rabbosa:
Rabbosa disagrees and says: "le'olam hu toreim min hayafeh" - one always separates from the finest. In his view, one must always separate from the very best, and if the kohen does not manage to receive it before it spoils, so be it; that is not the giver's concern. For the Torah asks that a person give from his finest as an expression of gratitude and thanks to Hashem, giving something back; and in this case he gives to the kohen, who serves as Hashem's agent. The emphasis, then, is on the giver and not on the kohen who receives.
The halachah: The ruling follows the Tanna Kamma - we separate whatever will be finest at the time the kohen receives it. If a kohen is readily available and common, one gives from the fresh fruit; and if a kohen is not readily available, one separates from the dried fig, which will keep until a kohen comes to his city to receive the Terumah.