Demai, Chapter 7, Mishnah 5. In the previous mishnah we dealt with someone who buys wine from Samaritans, which is certainly tevel, and with a person who discovers close to Shabbos, or on Shabbos itself, that Terumah and tithes were never separated from it. We learned that relying on the principle of bereirah he may designate portions of the wine as the future Terumah and tithes and drink the rest, since retroactively it will be established that what he drank was ordinary produce, and what he separates afterward is the Terumah and the tithes.
Along similar lines, our mishnah discusses someone who discovers close to Shabbos that he has produce from which Terumah and tithes were not separated, and he cannot reach it: the produce is sitting in his house while he is in the beis midrash or in the field, and he has no way to separate before Shabbos begins. What should he do?
First case - figs of tevel:
He should say: "Shtei te'einim she'ani asid le'hotzi u'lehafrish achar kach - harei hen Terumah" - "the two figs that I will later take out and separate are Terumah," and likewise he designates the remaining separations. Here is the order of designation out of a hundred figs:
Two figs out of a hundred - Terumah, which is one part in fifty.
Ten figs - and in practice slightly fewer than ten, nine point eight - ma'aser rishon.
The remaining nine - or slightly fewer than nine - ma'aser sheni.
On the basis of this designation he eats, and again, relying on bereirah.
Second case - figs of demai:
What if the figs are not tevel but demai, meaning that the Terumah has already been separated from them and the only doubt is whether the tithes were separated? We already learned earlier in the tractate that with demai one may separate on Shabbos itself, separating the tithes and eating the rest. Therefore on Shabbos he says:
"Mah she'ani asid lehafrish lemachar" - "what I will separate tomorrow," at the time I wish to eat, is ma'aser, referring to one hundredth of the total amount lying there.
And the rest of the ma'aser is adjacent to it - and that one hundredth which I designated as ma'aser at the outset will serve as terumat ma'aser for the rest of the ma'aser.
And ma'aser sheni is on its northern or southern side - and since we are dealing with demai, he is permitted to identify and fix the location of these portions.
"U'mechulal al hama'os" - the ma'aser sheni is redeemed onto the coins that I set aside.
And on the basis of this declaration he may eat the demai on Shabbos.
In summary: our mishnah taught how a person acts when he cannot separate before Shabbos. With tevel he designates in advance, by force of bereirah, which figs will be Terumah, ma'aser rishon and ma'aser sheni, and he eats the rest. With demai, where separation is possible on Shabbos itself, he designates the ma'aser, the terumat ma'aser taken from it, and the location of the ma'aser sheni, and he redeems it onto the coins.