Demai, Chapter Seven, Mishnah One
This mishnah provides a way for a person who does tithe to eat on Shabbat with a friend who does not tithe. The problem is that it is forbidden to tithe on Shabbat, so the one who does tithe will have to tithe on Friday the produce that he will eat on the next day.
One who invites his friend [before Shabbat] to eat with him [on Shabbat], and [his friend] does not trust him in respect of tithes, [the friend] may say on the eve of Shabbat, What I will set apart tomorrow, behold it shall be tithe, and what is nearest to it shall be the rest of the tithe. That which I made tithe will become the terumah of the tithe for the whole, and the second tithe is to the north or to the south and it shall be exchanged for money.
What the friend who tithes must do is separate on Friday the tithes from the food that he will eat on Shabbat at his friends home. We should note that he is separating tithes from demai, doubtfully tithed produce and not from tevel certainly untithed produce. This procedure would not work with tevel. Furthermore, since this is demai, he need not separate terumah because ame haaretz were not suspected of not separating terumah.
The first thing he does is declare that 1/100 of the food will be tithe, along with the other 9/100 that are adjacent to it. This makes 10/100 of the food tithe, as is required. The first 1/100 is then declared to be terumah for the tithe. Finally he declares that the second tithe is adjacent to the first tithe, either to the north or to the south. The second tithe he then declares redeemed for money.
On the next day he can now eat all of the food served to him, except for the part that he made into terumat maaser. He can eat the tithe because the food served to him was only demai, and as we have explained, tithes taken from demai may be eaten by their owner. He may eat the second tithe because he redeemed it for money.