Demai, Chapter Seven, Mishnah Five
In this mishnah a person has figs in his house that are either tevel (certainly untithed produce) or demai (doubtfully tithed produce). He is somewhere else, either in the study house (the Bet Midrash) or in the field and he is concerned that someone in his house might eat the produce before it has been tithed. The mishnah provides him with a way of tithing his produce without actually returning home.
1) He had figs of tevel in his house, and he is in the house of study or in the field: he may say: The two figs which I shall set apart shall be terumah, ten figs shall be first tithe, and nine figs second tithe.
2) If the figs were demai, he may say: Whatever I shall separate tomorrow will be tithe, and the rest of the tithe is adjacent to it. That which I made tithe will become terumat maaser for the whole, and the second tithe is to the north or to the south and it shall be exchanged for money.
Section one: In this section, he has tevel in his home and he is afraid that someone will eat it before it is tithed. To prevent this grave transgression, the rabbis provide a way for him to tithe the food before he even gets back home. Anything that they eat now will be tithed and he will (hopefully) get back in time to actually separate the terumah and tithes before they eat. We should note that because this is tevel, he doesnt have to separate terumat maaser, the terumah taken from the tithe. He will give the whole tithe to the Levite and the Levite will separate the terumat maaser himself, as is supposed to occur. Also noteworthy, the second tithe is only nine figs because he has already separated ten from the original one hundred and only ninety are left (actually 88 because he took two for terumah, but the mishnah is only approximating). So a tithe of 90 is only 9.
Section two: If the figs were demai, he makes the declaration slightly differently. First of all he distinguishes between the tithe and the terumat maaser, because he can keep the tithe for himself; he only has to give away the terumat maaser. Second, he can redeem the second tithe because when it comes to demai, one can redeem second tithe before actually separating the terumah and the maaser. In contrast, when it comes to tevel (certainly untithed produce) he cant redeem the second tithe until he has actually separated the terumah and the tithe, as we explained in yesterdays mishnah.