Demai, Chapter 6, Mishnah 11. This mishnah deals with the status of produce purchased in the region known as Surya, which is in present day Syria, and the question it addresses is what the law is regarding demai.
The special status of Surya:
Surya is not entirely equivalent to Eretz Yisrael in terms of halachah, because King David conquered it before he completed the conquest of all of Eretz Yisrael. For that reason, the sanctity of the Land never applied to it, and in many halachos it differs from Eretz Yisrael.
One of the halachos in which Surya differs concerns produce: whatever grows in Surya is obligated in terumos and maasros, and also in the laws of the seventh year and shemittah. Even so, one who buys produce in Surya need not separate the tithe of demai, since most of the produce sold in Surya comes from outside the Land, and even from outside Surya itself. So the basic status is that one who buys in Surya from someone who is not from Eretz Yisrael is exempt from separating demai. The question of the mishnah is what the law is when the produce is identified as produce that in fact came from Eretz Yisrael.
The first case - "Mishel Eretz Yisrael hen":
The mishnah states: "HaMocher peiros b'Surya v'amar mishel Eretz Yisrael hen - chayav l'aser" - someone who is not from Eretz Yisrael sells produce in Surya, and informs the buyer that this is not Syrian produce but produce that came from Eretz Yisrael. Since he stated this, the produce becomes obligated in demai and the buyer is required to tithe it.
But if at that very moment he goes on to say "Me'usarin hen" - that is, the produce indeed came from Eretz Yisrael, but the tithe has already been separated from it - then he is "ne'eman" (believed). At first glance this is puzzling: the man is an am ha'aretz, and he is not trusted regarding the separation of maaser, so his word cannot be relied upon. Why is he believed specifically here?
"SheHapeh she'asar hu hapeh shehitir":
The mishnah explains: "SheHapeh she'asar hu hapeh shehitir" - the mouth that forbade is the mouth that permitted. Had he said nothing, the produce would have been permitted, since whatever is sold in Surya is presumed to have come from outside Eretz Yisrael and outside Surya, and there is no need to separate demai from it. He himself is the one who identified the produce and forbade it by his words, and therefore that very same mouth that forbade is the mouth that permitted. Since the entire basis of the prohibition rests only on his statement, from the moment we believe him that the produce is from Eretz Yisrael, we can also believe him that the tithe has already been separated from it.
The second case - "Misheli hen":
If the seller said "Misheli hen" - the produce grew in his own fields or garden in Surya - then "chayav l'aser" - the buyer must tithe it, since produce that grows in Surya itself is obligated in maaser, as explained above. For ordinarily, when nothing else is known, we assume the produce came from outside Surya and is exempt; but this am ha'aretz informed us that the produce is from Surya, and Surya is obligated.
Here too, if he went on to say "me'usarin hen" - he is believed, for the same reason of "the mouth that forbade is the mouth that permitted": had he not spoken, we would have assumed the produce came from outside Surya and is permitted, and it is only his statement that he grew it in Surya that creates the obligation. Therefore, when he testifies that the tithe was taken, we can believe him about that as well.
The qualification - when it is known that he has a field in Surya:
But if it is known that this am ha'aretz has at least one field in Surya, the buyer must tithe, even if the seller says the tithe was separated. The reason: produce bought from an am ha'aretz who owns a field in Surya cannot be assumed to have come from outside Surya; rather, since he has a field in the area, we assume it came from his field and is obligated in maaser even without his statement. And since the obligation does not stem from his words, he is no longer believed when he says that the tithe was separated from it.
In summary: In this mishnah we learned that one who buys produce in Surya is exempt from demai, since most of what is sold there comes from outside Surya. But if the seller informed the buyer that the produce is from Eretz Yisrael, or that it grew in his own field in Surya, the buyer must tithe it. And if he added to his words that the produce has been tithed, he is believed, by the rule of "the mouth that forbade is the mouth that permitted," since the entire prohibition arises only from his words. All of this applies when it is not known that he has a field in Surya; but if it is known that he owns a field, the obligation exists even without his statement, and he is no longer believed when he says that the tithe was taken.