Demai, Chapter Five, Mishnah Five
This mishnah deals with buying produce from a poor person who goes begging from door to door. The problem is that he may receive his food from many different people, some of whom may have tithed and some who do not. As we have stated before, one cannot separate tithes from food that has already been tithed in order to exempt food that has not been tithed.
1) One who buys from a poor man, and similarly a poor man to whom they given slices of bread or pieces of fig-cake, he must tithe every piece.
2) But in the case of dates and dried figs he may mix them together and take [the tithes from the mixture].
3) Rabbi Judah said: When is this so? When the amount [of dates or dried figs] given to the poor man was large; but when the amount was small, he must tithe each separately.
Section one: One who buys from a poor man must be concerned lest every single piece of bread that he gets originates from a different person, and that some of it may have already been tithed. Therefore, he must tithe each slice of bread separately. The same is true, the mishnah notes, for the poor man himself. When he goes begging and receives slices of bread from various people who may themselves not separate tithes, he must separate tithes from each slice of bread separately.
Section two: The rabbis were lenient in the case of dates and dried figs because separating tithes from each and every one separately would be a rather large difficulty. Since this is only demai, there is room to be lenient.
Section three: Rabbi Judah limits this leniency to a case where the poor person received a large amount of dates or figs. In such a case it would truly be a hardship to separate from each date or fig separately. However, if there was not a large amount, the rabbis insisted that he tithe each fig or date separately. In other words, they were only lenient when tithing from each separately would have caused a lot of extra hassle.