(Mishnah - R. Li'ezer): Terumah is Batel in 101. R. Yehoshua says, in 100 and more (even less than 101).

There, R. Chizkiyah cited R. Avahu citing R. Lazar to say that wherever Rebbi taught an argument [in the Mishnah] and afterwards a Stam Mishnah, the Halachah follows the Stam. (Since our Stam Mishnah says 101, like R. Eliezer, this is the Halachah.)

Rebuttal: Since [the coming] Mishnah taught 'a Se'ah of Terumah that fell into 100 [of Chulin]', and R. Lazar said that the Mishnah does not say 'into 100', rather, 'into 99' (and another half-Se'ah joins with the Chulin to be Mevatel the Terumah, this is like R. Yehoshua), it is as if there are Stam Mishnayos, [one like] R. Li'ezer and [one like] R. Yehoshua (so we cannot learn from here whom the Halachah follows. We explained this like MAHARA FULDA.)

Question: What is the source that they are Batel [in 101]?

Answer (R. Yonah): It says "mi'Kol Chelbo Es Mikdesho Mimenu'' - what you are Merim from it (separate for Terumas Ma'aser), if it falls into it, it is Mekadesh (forbids) it. How much is this? [Terumas Ma'aser] is one part in 100.

R. Li'ezer says, he adds a Se'ah and it is Batel [in 101]. R. Yehoshua says, he adds any amount, and it is Batel [in more than 100].

R. Yosi ben Meshulam says, he adds a Kav to 100 Se'ah, a sixth of the Meduma (the amount of Terumah that fell in).

(Beraisa - R. Shimon): Terumah is Batel in 100 from a Kal v'Chomer;

If Isur amidst Isur is Oleh (one removes one part of Terumas Ma'aser from 100 parts of Tevel, and this permits it), Isur amidst Heter (Terumah that fell into 100 parts of Chulin), is it not all the more so Oleh (Batel)?!

Terumah - some teach that it is [called] Isur amidst Heter (like our text of the Beraisa of R. Shimon), and some teach that it is [called] Heter amidst Heter;

The one who says Isur amidst Heter, for Terumah is forbidden to Zarim. The one who says Heter amidst Heter, for Terumah is permitted to Kohanim.

Some explain all of it regarding Kohanim. The one who says Isur amidst Heter, discusses Tamei Terumah. The one who says Heter amidst Heter, discusses Tahor Terumah. (We explained this like GRA.)