Gittin, Chapter 3, Mishnah 8. This mishnah continues to deal with the subject of chazakah - the presumption that a given thing still exists where it was left.
"HaManiach peiros lihyos mafrish aleihen terumah u'ma'asros, ma'os lihyos mafrish aleihen ma'aser sheni":
"HaManiach peiros" - a person who sets aside a quantity of produce or grain and designates it to serve as Terumah and ma'asros for other produce in his possession. That is, he has grain in hand, and he determines that its Terumah and ma'asros will be taken from that produce which he left in a particular place.
"Ma'os lihyos mafrish aleihen ma'aser sheni" - money set aside and designated for redemption, upon which he will redeem the ma'aser sheni in his possession.
What both cases have in common: the person is not presently at the place where the produce or the money is lying.
"Mafrish aleihen b'chezkas shehen kayamin" - he may separate Terumah and ma'asros relying on the produce he set aside, and redeem the ma'aser sheni relying on the money he set aside, on the assumption that they are still there and nothing has happened to them.
A note regarding separating from the mukaf:
There is a halachah that one may separate Terumah and ma'asros only min hamukaf, meaning that the produce from which one separates must be near the produce for which one is separating. There are, however, exceptions, such as on Erev Shabbos and Erev Yom Tov, when one may be lenient and do so. Others explain that the mishnah is dealing with something that should not be done l'chatchilah, yet after the fact the separation takes effect - and it is such a case that is under discussion here.
"Im avdu - harei zeh chosheish me'eis l'eis, divrei Rabbi Yehudah":
If it was discovered that the grain designated for Terumah and ma'aser, or the money designated for redeeming ma'aser sheni, was lost and is no longer in its place - the question arises how far back one must be concerned that it was already gone. In the opinion of Rabbi Yehudah, the concern extends me'eis l'eis: twenty four hours before the moment of discovery. In other words, during that twenty four hour period we must assume that it was already lost, while before that we may assume that it was still in its place.
The Gemara notes that the other Sages disagree with Rabbi Yehudah and hold that one must be concerned going back to the point at which the matter is clearly known otherwise - that is, once it was discovered that they are not in their place, we assume that they were not there since the last moment at which it is known with certainty that they were.
"B'shloshah perakim bodkin es hayayin":
Rabbi Yehudah adds here a matter that does not bear directly on this point, though it has a certain connection to it. A person who set aside wine in order to separate Terumah and ma'aser from it for other wine - there are three times in the year at which he must inspect the wine and verify that it is still wine and has not turned into vinegar, since one does not separate Terumah and ma'aser from vinegar for wine, but only from wine for wine.
These are the three times:
"B'kadim shel motza'ei hechag" - at the time when the east wind blows, at the conclusion of the festival of Sukkos.
"B'hotza'as semadar" - when the blossom falls off and the tiny fruit of the vine appears.
"Bish'as knisas mayim b'voser" - when the liquid begins to gather within the grapes themselves, while they are still unripe.
In summary: In this mishnah we learned that one who sets aside produce for separating Terumah and ma'asros, or money for redeeming ma'aser sheni, may separate upon them under the presumption that they still exist; and if it was discovered that they were lost, Rabbi Yehudah holds that the concern extends me'eis l'eis, while the Sages hold that the concern extends back to the point that is clearly known. We also examined the three times at which one inspects wine designated for separation, lest it have turned to vinegar.