Demai, Chapter Six, Mishnah Eleven
In rabbinic language Syria refers to the land that borders the land of Israel to the North and East but is not considered fully part of Israel. The rules of tithing and terumah do apply to produce grown by a Jew in Syria but one who purchases produce in Syria can assume that it grew on gentile land and is therefore exempt from the laws of tithing and terumah. Our mishnah deals with a person who is buying produce in Syria and with the question of whether or not he has to separate tithes.
1) One who sells produce in Syria and declared It was grown in the land of Israel, the purchaser must tithe it.
2) [But if he also said], It has been tithed, he may be trusted, because the mouth that forbade is the same mouth that permitted.
3) [If he said: The produce is] from my own field, the purchaser must tithe it.
4) [But if he added:] It has already been tithed, he may be trusted, because the mouth that forbade is the same mouth that permitted.
5) If it was known that he had a field in Syria, the purchaser must tithe it.
Section one: If the seller says that the produce was grown in the land of Israel then the purchaser must tithe it. The status of such produce is demai for the seller may indeed be telling the truth.
Section two: However, if the seller also says that he tithed the produce, he is believed. This is based on the principal the mouth that forbade is the same mouth that permitted. When the seller said It was grown in the land of Israel he forbade the produce. Had he not said anything, it would have been permitted under the assumption that the produce grew in Syria in a gentiles field. When he says, It has been tithed he is permitting. Since he was believed to prohibit something, he is also believed to permit it.
Section three: If the seller is a Jew and he says that the produce was grown in his own field here in Syria, the purchaser must tithe the produce, as we explained in the introduction.
Section four: If the seller then adds that he already tithed the produce, he is believed under the same principle employed in section two. He forbade the produce (until it was tithed) by saying that it grew in his field, and he is the same one permitting the produce by saying that he had tithed it.
Section five: The principle of the mouth that forbade is the same mouth that permitted only works when there is no external reason to forbid. If we know that the seller was not just a producer seller but also owned a field in Syria, then we have reason to think that the produce grew in his land. If it grew on his land then it is liable to be tithed. In this case, it was not his mouth that forbade but rather our knowledge that he owned a field which made the produce liable for tithes. Since it was not his mouth that forbade, he is not believed to permit.