Baraisa (R. Eliezer): Terumas Maaser of Demai that fell back into the wheat in the place from where they came, it makes it Meduma (a mixture of more than one part Terumah to 100 parts non-Terumah - that may not be eaten by non-Kohanim). If it didn't fall into its place (meaning that it fell into other fruit), it doesn't make Meduma.

(Chachamim): Whether it fell into its place or into another place, it makes Meduma.

(R. Shimon): Whether it fell into its place or whether it didn't fall into its place, it doesn't make Meduma.

Question (R. Bun bar Chiya to R. Zeira): Both the Chachamim and R. Shimon are logical as they don't differentiate between whether it fell into its place or another place. But according to R. Eliezer, why if it fell into its place does it make Meduma and into another place it doesn't make Meduma?

Just as they were discussing R. Bun's question, R. Chagai entered the Beis Midrash. They said, "Certainly, when R. Chagai will be asked this question, he will respond by saying - 'I swear by Moshe that this is the reason...'". And that is in fact what he said...Why does it make Meduma in its place? Because if it would fall back to the same place and become permitted, people would question the purpose of having separated it originally in order to permit the remainders. But when it falls into different fruits, people wouldn't have this question.

R. Ila ruled like this teaching of R. Chagai.

(R. Zeira): Such a case came before R. Chanina and he ruled like R. Shimon Shazuri (above Zevachim 105(b)).

(R. Acha citing R. Yonasan): The Halacha does not follow R. Shimon Shazuri. Not only if it fell in its place, but even R. Shimon Shazuri says that if he separated Terumas Maaser and the remaining fruits became burnt, he may ask the seller and rely on his answer to eat the Terumas Maaser.

When R. Zeira said (that the Halacha is like R. Shimon Shazuri, he was only speaking about his ruling in the Mishnah, but) not this case (of the fruits burning).

A case came before R. Chanina and he ruled like R. Shimon Shazuri.

(R. Avin): The first case of the Mishnah (asking the seller on Shabbos and relying on him to eat) is unlike the second case (if the Terumas Maaser mixed back in, even during the week, R. Shimon Shazuri said the he can ask the seller and rely on him to eat). When he asked in the first case, it isn't because he's accusing him of being someone who doesn't tithe - rather he asks him in an indirect manner, saying, "I saw that you were selling wheat to many people and I was concerned that you forgot to tithe". And if the seller answered that they are tithed, he can rely on this. But in the second case, he suspects the seller and asks him directly if he tithed and the seller answered that he had. Can you rely on this? Maybe he's lying! This is why R. Acha ruled against R. Shimon Shazuri.

(R. Shmuel son of R. Yosi bei R. Bun): Bar Kapara also ruled like R. Shimon Shazuri, that he may rely on the word of the seller when the Terumas Maaser fell back in, because the fear of the prohibition of eating Meduma is upon him and he will tell the truth.

(Mishnah): If a man imposed a vow on his friend to eat with him and the friend does not trust him over Maaseros, he may eat with him the first Shabbos (after his marriage), as long as the host told him that it was tithed. And on the second Shabbos, even though there is a vow imposed on him if he doesn't eat with him, the friend still may not eat with him unless he (himself) tithes.

(Gemara) (R. Yannai bei R. Yishmael citing R. Yochanan): They permitted relying on the host during the Shabbos Sheva Berachos (the first Shabbos of the marriage) in order to avoid animosity (see above Zevachim 106 (e)).

(R. Avun): Here, they permitted Rabbinic Tevel for the sake of peace.

Question (R. Chanina citing R. Yirmiyah): If it is for the sake of peace, why must the host say that it has been tithed?

Question #2: If he was invited for that first Shabbos but he didn't come, on the second Shabbos, may he eat as if it's the first Shabbos? (The Gemara does not answer these questions.)

Rav Chisda: (From this that the Gemara did not permit the second Shabbos) we learn that it is prohibited for a Chaver to eat from a meal unless it is a Seudas Mitzvah.