Mishnayos Terumos Perek 4 Mishnah 2

במגורה – in a storehouse, as it is written (Haggai 2:19): “While the seed is still in the granary, [and the vine, fig tree, pomegranate and olive tree have not yet borne fruit].”

ונתן סאה לבן לוי – Our Mishnah is speaking about a worker who is a member of the order for the observance of Levitical laws in daily intercourse (i.e., Haver), who comes to dine with the owner of he house and does not believe him on the tithes, and the worker saw that owner of the house giving from his storehouse a Seah to the son of a Levite (i.e., First Tithe) and a Seah to a poor person (i.e., the Poor Tithe, in years 3 and 6 of the seven year cycle). Rabbi Meir states that this worker can eat from the storehouse eight Seah, for undefined, according to the law of First Tithe, he gave to the Levite, and according to the law of the Poor Tithe, he gave to the poor person, and behold there are eight Seah which were already made legally fit for use by giving the priestly dues and a little bit more. And even though he did not see him separate the Great Terumah (i.e., the two-percent for the Kohen), the illiterate were not suspected [of not fulfilling] the heave-offering.

וחכ"א לא הפריש בעל הבית אלא לפי חשבון (with a calculation [of the percentage of tithes which remain to be separa from the batch as a whole]) - this meal that the worker wants to eat with him., but the remainder is a mere gift that he gives to the Levite and the poor person, therefore, the worker does not have to eat with him until he has completed eight Seah according to the words of Rabbi Meir, but that meal which he was to eat with him and nothing more. Another explanation: “and he gave a Seah to the son of Levite and a Seah to a poor person, and they went on their way. But when he came to separate eight Seah from the storehouse and to eat them on the support of the two Seah that he gave to the son of the Levite and the poor person, he doesn’t know if those two Seah exist or not. Rabbi Meir says that he should separate eight Seah and eat them, in support of those two Seah that he gave to the son of the Levite and the poor person in the presupposition that they exist. But the Sages say that he does not separate other than according to a calculation [of the percentage of tithes which to be separated from the batch as a whole], that exist; if all of the two Seah exist, he separates upon them eight Seah and a bit more, and if only one-half of them exist, he does not separate on them other than four Seah, according to the calculation of what he finds of them existing, thus he separates upon them and not more. And the Halakha is according to the Sages.