Terumos, Chapter 4, Mishnah 1. This mishnah takes up a fascinating question: what is the status of a person who separated only part of the Terumah he is obligated to give, intending to complete the required amount at a later time.
The case:
A person has a hundred cups of wine from which Terumah must be separated. Since the measure of Terumah is one in fifty, he needs to separate two cups out of the hundred. A kohen comes and knocks on his door and asks, "Do you have Terumah?" - the owner takes one cup and gives it to him as Terumah, but he does not separate the second cup at that moment. He leaves his container of tevel partially tithed, intending to complete the separation later on. The question is: may he come back the next day and take a second cup of wine from the ninety-nine that remain?
The initial assumption would be that, since he has already taken one of the two required cups, of the ninety-nine that remain, forty-nine are chullin from which Terumah has already been separated, and fifty are still tevel that requires separation. There is therefore room to worry that when he takes the second cup, it will not come from the fifty of untithed tevel but specifically from the forty-nine of chullin that have already been tithed.
But this line of thinking is mistaken. The mishnah states clearly that there is no problem here at all, and no reason why he should not come back the next day and take a second cup. The reason is that this is considered the middle of a single process: just as a person takes one cup and a moment later takes a second cup - which is the necessary way to separate Terumah in many cases - and we do not worry that the second cup is coming from the portion that has already been tithed, so too here. This is one long process, and as long as it is one continuous process, it makes no difference how long it takes, and we do not worry that the person is taking again from something whose Terumah has already been separated. Therefore, just as he may take one cup and a second cup a minute later, so too he may take the second cup a day or a week later.
The words of the mishnah:
"HaMafrish miktzas Terumah o maaser" - one who separates part of the Terumah or part of the maaser, "gomer alav" - he may come back a second time and separate the remainder of the Terumah or the maaser, as the case may be, with no problem at all.
"Aval lo lemakom acher":
Here the mishnah adds a qualification. True, he may come back and take the second cup from the ninety-nine that remain, but he may not say: I will use the ninety-nine that remain and take two cups from them - one to complete the Terumah of that same barrel, which was a hundred and is now ninety-nine, and the second for a different barrel of mine that holds fifty cups of wine. Such a separation does not work, because the mixture of ninety-nine is considered half tithed: it stands in the middle of the tithing process and carries only a partial obligation rather than a full one, unlike the fifty that have not been touched at all, which require full tithing and are not tithed even in part.
The result is that he may take the second cup from the ninety-nine on behalf of those same ninety-nine, but he may not take a second or third cup from the ninety-nine on behalf of a different container of fifty - since the two are not on the same level.
The view of Rabbi Meir:
"Rabbi Meir omer: af motzi hu lemakom acher Terumah umaaser" - Rabbi Meir disagrees and holds that the two are on one level. Whereas the first tanna views the mixture as half tithed, Rabbi Meir holds that there is no such thing as half tithed: either something is tithed or it is not. And as long as its tithing has not been completed, it is fit to serve as a source for Terumah and maaser even on behalf of something that was never tithed at all, for the container of ninety-nine still has available Terumah in it, and one can take from it whatever is needed - one cup or two and so on.
In practice: The halachah follows the first Tanna. A person may go back a second time and take the second cup on behalf of the remaining ninety-nine, but he may not take a second or third cup from those ninety-nine on behalf of a separate batch of fifty that was never touched.