From Tosfos d"h Rabbi Eliezer, as well as Rashi d'h h"g Rabbi Nasan, it's clear that they assume that Rabbi ELiezer agrees that if you are meyached something for shabbos, it becomes nikva for chiyuv maaser and is assur to eat even after shabbos, and R. Eliezer is only meikel here because he wasn't meyached it for shabbos. But lechora it seems like R. Eliezer would disagree and hold that even if you're meyached for shabbos, it would be mutar after shabbos. Because R. Eliezer holds mosro chozer is not nikva, and if you're meyached for shabbos, it's like being meyached for an achilas keva, but R. Eliezer holds even if you actually did meyached for an achilas keva, even with a maaseh, it still is not kovea for maaser, so why would being meyached it for shabbos be more chamur than a case of mosro chozer. SO why do Rashi and Tosfos assume that R. Eliezer agrees to that braisa of tinokos shtamnu, why not just say he disagrees? (After all, he disagrees apparently with the mishna on 3
4b about hamaavir teaynim bechatzeiro, because he only allows you to eat it after you remove it from the chatzeir, while that mishna allows it to be eaten in the chatzeir as well. So why not just say he also disagreees with the braisa there?)
Yaron Barach, Brooklyn, USA
I am going to write a rather short answer for the moment. Apologies for the brevity.
1. It seems that Rebbi Eliezer maintains that the Din that Shabbos is Kove'a for Ma'aser is stronger than the Din of Mosro Chozer for "Achilah Merubah" on a weekday. This is apparent from Rashi (35a, DH v'Halo) and Tosfos (35a, DH v'Halo), who write, "even if he takes a lot of the Tevel on a weekday." This implies that it was clear to Rashi and Tosfos that Mosro Chozer applies only on a weekday. Rashi writes "d'Mechzi d'Kevi'a" -- it looks as if it is Kavu'a. This seems to imply that it only looks as if it is Kavu'a, but perhaps mid'Oraisa it is not Kavu'a. In contrast, the Din that Shabbos is Kove'a for Ma'aser is learned from a verse by the Gemara here (34b).
2. Tosfos (35a, DH Rebbi Eliezer) stresses that in the Mishnah of Tinokos she'Tamnu (by the way, this is a Mishnah, not a Beraisa, which is another reason why it is not so simple to say that Rebbi Eliezer disagrees with it) there was Yichud. The Pnei Yehoshua writes that since he was Meyached, this gave them greater importance. In contrast, it is clear from Rashi (35a, DH v'Halo) that the case of Mosro Chozer is not referring to a case of Yichud but merely to a scenario where it looks as if there has been Kevi'us.
3. The Rashba (near the end of the Perek in the Gemara) writes that the Mishnah of ha'Ma'avir Te'enim b'Chatzero follows the opinion of Rebbi Eliezer, but he only permits eating the figs outside the Chatzer.
Kol Tuv,
Dovid Bloom