(R. Zeira/ R. Acha/ R. Tanchum b'R. Chiya citing R. Yehoshua ben Levi): There are 3 cases where one should tithe without the knowledge of the owner (as the obligation to tithe is Rabbinic, and the owner is not trusted to do it himself) - 1) one whose Tevel became mixed with (a majority of ) Chulin (tithed produce); 2) one who buys from a group of Kusim; and 3) Terumah of Chutz LaAretz (produce).

(R. Zeira): These (cases 2 &3) teach that he may not take money from the tribe (of Levi) (for giving them tithes from buying from a group of Kusim or when separating Terumah of Chutz LaAretz) - as if he could, it would be like separating Terumah with permission.

(R. Yochanan): R. Eliezer's reasoning (in our Mishnah Zevachim 118 (b), for prohibiting tithing for a Kusi's produce for a Kusi's produce) is - Even though most Amei HaAretz tithe, they said that the fruits of an Am HaAretz are Demai because of the minority and one may not separate Terumos and Maaseros for one on another. So too, even though most Kusim do not tithe, they ruled that they are Demai and one may also not separate Terumos and Maaseros for one on another.

(R. Bun bar Chiya citing R. Zeira): (The Mishnah taught that one may not tithe from a planter without a hole for one with a hole.) They were doubtful as to the status of a planter (without a hole).

Question (Mishnah in Kilayim): 'Planting in a planter with a hole inside a vineyard prohibits (as it's considered to be planting); without a hole does not prohibit (as it's not considered to be planting).' Surely it should become prohibited from doubt? (The Gemara leaves this question unanswered.)

Question: Just as the Mishnah taught that if one separated from definite Tevel for Demai, it is Terumah, but he may not eat from that Terumah until he separates Terumah and Maaser for it (as perhaps the Demai was already tithed); so this should apply if he separates from Demai for Demai?

Answer (R. Bun bar Chiya citing R. Shmuel bar R. Yitzchak): The Terumas Maaser that he gave from Demai for Demai is still a doubt; so if we obligate it to be tithed, it is considered like doubtful tithing of Demai, for which one may be lenient - just as doubtful Dimuah is exempt from Demai. (Dimuah is a Chulin mixture in which a small amount of Terumah fell. Since there is not enough Chulin to negate it, the whole mixture becomes doubtful Terumah. And if it is doubtful Dimuah, it is exempt from Demai.)