Demai, Chapter Seven, Mishnah Four
While Samaritans did seem to tithe the produce that they ate, according to the rabbis they did not tithe that which they sold. Therefore, one who buys produce from them will have to tithe it himself. In the case in our mishnah, the person is in Samaria, the northern part of Israel, and he wants to buy wine from the Samaritans, drink it there, and separate the tithes later. In this way he will avoid having to carry the tithes and terumah with him in separate containers so that when he returns to his home he can give them to the priest and Levites. The mishnah provides a means for him to separate tithes in the future from the wine which he is drinking nowthink of this as trading in futures, or if youre into sports, trading for a player to be named later.
I should note that I have explained this mishnah according to Albeck. Others explain the mishnah as referring to someone who wants to separate tithes on the eve of Shabbat for wine that he will get on Shabbat.
One who buys wine among Samaritans, he may say: Two logs which I shall set apart shall be terumah, ten logs tithe, and nine logs second tithe. He may then pour the wine and drink it.
In general what the person does is declare that he will separate the terumah and tithes in the future from the wine that he is drinking now. This will allow him to drink the wine and only later take out the terumah and tithes. The first thing that he does is declare that he will separate 1/50 of the wine, that is 2 logs out of 100, for terumah. Then he declares that the next ten logs will be tithe, and that after that, the following nine logs, which is about ten percent of that which is left, will be second tithe. Note that in this case, because the produce is actually tevel, that is certainly untithed produce, he does not redeem the second tithe before he separates it. In the previous examples of similar types of arrangements, such as those in mishnayot 1-3, he was separating tithes from demai and hence he could redeem the demai immediately.
After having declared that he will separate the necessary tithes and terumah in the future, he may now pour the wine and drink it.