Mishnah 9 in chapter 1. In the previous mishnayos we learned that when we have before us two batches of produce, one of which has reached the completion of its processing and the other of which has not yet reached that stage, we do not take Terumah from one for the other at all: not from the finished for the unfinished, and certainly not from the unfinished for the finished.
The novelty of our mishnah:
Our mishnah teaches that when we have before us two types of food and both have reached the completion of their processing, even though they are in different states, one may take Terumah from one for the other. The simple case is:
Wine and grapes - when the owner does not intend to turn the grapes into wine, he may take Terumah from the wine for both the wine and the grapes together.
Oil and olives - when he does not intend to press the olives into oil, he may take Terumah from the oil for both the oil and the olives together.
The more complex side of the mishnah:
The mishnah permits taking Terumah from a second vessel containing olive oil for olives that are lying in a first vessel and are destined to be pickled. Since those olives are not going to be pressed into oil, they are considered finished, and therefore one may take oil as Terumah for both of them - for the oil and for the olives. Similarly, if the second vessel contains wine and the first vessel contains grapes that the owner intends to leave to dry and become raisins, and he does not intend to make them into wine, one may take Terumah from the wine in the second vessel for the grapes in the first vessel.
At first glance one might wonder: grapes are not raisins, and raw olives are not pickled olives, so it turns out that there is still work left to be done. Why, then, is this considered the completion of the processing?
The Yerushalmi answers that those stages of finishing the produce are not considered a significant amount of work, since they happen almost by themselves: the raisins dry on their own, and the olives pickle on their own - all that is involved is putting them into a jar and the like. Since this is not significant work, it does not prevent taking Terumah from wine or oil whose processing has already been completed.
Why may one not take Terumah in the reverse direction?
Note that it was not permitted to take Terumah from raisins, or from grapes destined to become raisins, and likewise from pickled olives or olives destined to be pickled, for the wine or for the oil. The reason is that the wine and the oil are of finer quality and more valuable, more work has been invested in them, and the kohen would prefer them - and we may not cause a loss to the kohen. Therefore one is obligated to take Terumah from the superior item: one may take oil as Terumah for olives that will never be pressed, and wine for grapes that will never be pressed, but not the reverse - one does not take finished grapes or finished olives as Terumah for the wine or for the oil respectively.
One who separated and afterwards changed his mind:
The mishnah continues and discusses one who has already taken Terumah in one of the following ways:
"Shemen al zeisim la'achilah" - he separated oil for olives that were designated to be eaten as they are and would never be pressed.
"Zeisim al zeisim la'achilah" - he had two batches of olives before him, and he had no intention of pressing oil from them, and he separated Terumah from one batch for the other.
"Yayin al anavim la'achilah" - he separated wine for grapes that were designated to be eaten as grapes.
"Anavim al anavim la'achilah" - he separated grapes for grapes that were designated to be eaten.
All of these are valid, as already emerges from the first part of the Mishnah, since in every one of these cases the processing has been completed.
But the Mishnah adds a further point: "V'nimlach l'darkan" - after he separated the Terumah he changed his mind and decided that in the end he would press those grapes or olives. Must he go back and separate Terumah a second time? The Mishnah rules: "Eino tzarich lachzor v'litrom" - he need not separate Terumah again. The reason is that the Terumah he separated was valid at the time he separated it, and once the Terumah has already been separated and what remains in his possession is ordinary produce, there is no need to separate Terumah again, and there is no problem with what he did.
In summary: Our Mishnah teaches that two types of food whose processing has been completed may be used to separate Terumah one for the other, even though they are in different states. Olives designated to be pickled and grapes designated to be made into raisins are also considered finished, because the processing that still remains happens on its own. Terumah is taken from the superior item - oil for olives and wine for grapes, and not the reverse, because of the kohen's loss. And finally, one who separated Terumah properly and afterward changed his mind and decided to press the fruit - his Terumah stands, and he need not separate Terumah again.