Terumot, Chapter Two, Mishnah Four
This mishnah deals with the rule that one cannot give terumah from one kind of produce on behalf of another. The second half of the mishnah deals with giving the terumah to the priest.
1) They may not give terumah from one kind for another kind, and if he did give, the terumah is not terumah.
2) All kinds of wheat count as one.
3) All kinds of fresh figs, dried figs and fig cakes count as one, and he may take terumah from one for the other.
4) Wherever there is a priest, one must give terumah from the very best, and where there is no priest, one must give terumah from that which lasts longest.
a) Rabbi Judah says: he should always give only from the very best.
Section one: One cannot give terumah from any species that is kilayim (mixed seed) with another species (we will see this in mishnah six, below). And if one does try to give terumah from one species on behalf of another that is kilayim with it, the terumah doesnt count.
Section two: Different species of wheat, such as white wheat and red wheat all count as wheat and one can give terumah from one on behalf of the other.
Section three: Similarly, all kinds of figs can be used for terumah for other kinds of figs. This is because all of these kinds of figs have, at least potentially, finished their processing.
Section four: The mishnah now turns its attention to what kind of terumah one should optimally give to the priest. If the priest is right there when the person is separating the terumah, he should give him the best of the produce. If he is not right there, then he should give him the part of the produce that will last the longest. For instance, if he has fresh figs and dried figs, if the priest is right there he should give him the fresh figs, but if he is not around, he should give him the dried ones.
Rabbi Judah says he should always give him the best of the produce and we are not concerned lest the produce goes bad.