(R. Yitzchak bar Eliezer): On Erev Shabbos but not on Shabbos, a person may declare, "This will be Terumah tomorrow".

(R. Yosi bei R. Bun): He may not do this on Erev Shabbos.

Question: There's a Mishnah (in Maseches Tevul Yom) that disagrees with R. Yosi bei R. Bun - If a jug became Tamei and it was immersed in a Mikveh, it has the status of Tevul Yom (and it is still considered Tamei until sundown). If it was filled from a barrel of Maaser Rishon before its Terumas Maaser had been separated in order to declare the contents of the jug Terumas Maaser - if he said, "This should be Terumas Maaser at nightfall", it is valid Terumah. If he then said, "Make for me an Eiruv Techumin with this food", it is invalid (since this produce will be Tevel until Shabbos itself and an Eiruv must be made before Shabbos and cannot be made with Tevel)...?

Answer: It is not ideal from him to declare (that it should be Terumas Maaser at nightfall), but if it was done, it is valid.

Question: But R. Chiya taught in a Baraisa that 'He says', which sounds like it is ideal; not only if it was already done?

As a result of this question, R. Yosi retracted.

(Mishnah): If he has Tevel figs in his home and he is in the Beis Midrash or in the field (and he does not have time to go back home and physically tithe before Shabbos), he should say, "The two figs that I will separate will be Terumah; ten shall be Maaser and nine shall be Maaser Sheni". (This solution relies on the principle of Bereirah - retroactive selection.)

If the figs were Demai, he should say, "Whatever I will separate tomorrow is Maaser and whatever is closest to it shall be the rest of the Maaser; that which I made Maaser shall become Terumas Maaser for it and its Maaser Sheni shall be to the north or south of it and redeemed onto money.

(Gemara): The case must be that the tithing takes effect from now (and works based on the concept of Bereirah - retroactive selection).

Imagine that the fruits were Tahor and that if he would separate the Terumah, it could be given to the Kohen to eat on Shabbos - so that he should say, "It is already Terumah and when I eat the fruits, (it will be identified)."

And imagine if the Terumah was Tamei, he should say, "It is already Terumah and when I will place it in the corner, (it will be identified and given to the Kohen to burn after Shabbos)."

Baraisa: He may carry even Tamei Terumah on Shabbos.

(R. Zeira): We learn from here that if Tevel has a stipulation on it, it may be carried on Shabbos. What should he do with the Tamei fruits? He should decide that a particular part will be Terumah and he may then eat the rest.