(Ulla bar Yishmael citing R. Yochanan): A person can exempt his Tevel with one Seah of Tevel. How is this? He brings a Seah of Tevel and makes it the Maaser Sheni. He then redeems it and then makes it into Terumas Maaser for its (or other) Maaser Rishon. (This shows that he reasons that it is possible to declare Maaser Sheni before Maaser Rishon.)

Ulla went to Bavel and he said this in the name of R. Yochanan and the people there came together to question it...

Question (Rav Sheshes): The Mishnah (further on) disagrees with this - If a person had two baskets of Tevel and he said that the Maaseros of A should be in B and vice-versa; A becomes tithed but B does not. The Tosefta explains - He takes 2 figs (as Terumah) from B, and 2 tenths (meaning Maaser Rishon and Sheni) and a tenth of one of the tenths (as Terumas Maaser) According to the teaching of R. Yochanan, why not take two figs and declare them Maaser Sheni and then redeem them and then make them into Terumas Maaser?

(R. Mana): (Further explaining the question) The Maaser Sheni in the second basket (which is separated for the produce in the first basket) still hasn't has its own Maaser Rishon taken! (But according to R. Yochanan, you should be able to redeem it and then turn it into Maaser Rishon?!)

Answer (R. Chananyah): There, both baskets were Tevel and he needs to separate all of the tithes from the first basket to fix up both baskets. Here, the Maaser Rishon is already taken and he is now coming to take Maaser Sheni, so after the Maaser Sheni is separated, he can then make it Maaser Rishon to be able to separate Terumas Maaser.

The Mishnah taught: The oil that a weaver rubs between his fingers is obligated in Demai; but the oil that the comber adds to his wool is exempt from Demai.

What is the difference between these two cases? On the weaver's fingers, it is absorbed as if he ate it, so it is obligated. The oil is absorbed into the wool, but not into the comber's body, so it is exempt.