Demai, Chapter Four, Mishnah Four
This mishnah deals with the prohibition of separating terumah and tithes on Shabbat.
1) One who had designated the terumat maaser of demai, or the poor mans tithe of produce that had certainly not been tithed, he should not separate them on Shabbat.
2) But if the priest or the poor man regularly ate with him, they may come and eat provided that he informs them.
Section one: In the scenario described in this mishnah, during the week a person had designated that a certain portion of his demai produce would be terumat maaser (the terumah taken from the tithe) or that a certain portion of his certainly untithed produce would be poor mans tithe. He had not yet separated these gifts from the rest of the produce. Even though he had already designated them, he may not actually separate them on Shabbat in order to give them to the priest or to the poor person. Again, one is not allowed to separate terumah or tithes on Shabbat.
The mishnah uses the example of poor mans tithe taken from produce that had certainly not been tithed and not from demai (which might have been tithed), because if the produce had been demai, he would not have to give the tithe to the poor person, as we learned in yesterdays mishnah.
Section two: If there was a priest or a poor person who regularly comes to his home to eat with him, then they may come on Shabbat and take the terumah or poor mans tithe that has already been designated as such. However, the owner of the produce must inform the priest or poor man that what they are taking is terumat maaser or poor mans tithe. If he does not, he has not fulfilled the mitzvah of giving these gifts to the priest or the poor person.