(Mishnah): One may not buy Terumah with Ma'aser Sheni money, since he is reducing its ability to be eaten (as only Kohanim may eat Terumah). R. Shimon permits it.

(R. Shimon to Chachamim): If it's permitted to buy an animal for use as a Shelamim offering, even though it could bring it to the prohibitions of Pigul, Nosar and Tamei; should we not be lenient with Terumah?

(Chachamim to R. Shimon): Although we can be lenient with Kodshim because they are permitted to non-Kohanim; how can we be lenient with Terumah, since it's prohibited to non-Kohanim?!

(Gemara) (Baraisa): (An additional reason to prohibit buying Terumah is that) it might become disqualified (and prohibited to eat).

Question: How might it become disqualified?

Answer (R. Yona): With a Tevul Yom (that disqualifies Terumah but not Ma'aser Sheni). It cannot be eaten because on a Torah level, it's Terumah Temeyah. It cannot be redeemed because on a Torah level, concerning Ma'aser Sheni, a Tevul Yom doesn't disqualify it from being eaten.

(The Mishnah taught that one may not buy Terumah with Ma'aser Sheni money, since he is reducing its ability to be eaten.) What does this mean? Terumah is prohibited to non-Kohanim, but Ma'aser Sheni is permitted. Terumah is prohibited to a Tevul Yom, but Ma'aser Sheni is permitted.

Question: (Why did the Mishnah discuss only the Ma'aser Sheni's reduction in ability to be eaten?) Just as he reduces the Ma'aser Sheni's ability to be eaten, so too he reduces the Terumah's ability to be eaten, as Terumah is permitted to an Onen (a person who is prior to or on the day of burial of a close relative), but Ma'aser Sheni is prohibited. Furthermore, Terumah need not be eaten in a specific place, but Ma'aser Sheni must be eaten in Yerushalayim...?

In fact, there is a Baraisa that teaches that buying Terumah with Ma'aser Sheni reduces both the Ma'aser Sheni's and the Terumah's ability to be eaten.

(Baraisa): One may not buy Sheviis produce with Ma'aser Sheni money.

(R. Yosi): This depends on the disagreement (in our Mishnah between R. Shimon and Chachamim over buying Terumah with Ma'aser Sheni).

(R. Yona): (Disagreeing) Both agree here. (R. Shimon was only lenient with Terumah because) those who eat Terumah are careful (that it won't become disqualified).

Question (R. Chunia to R. Mana) (Mishnah in Maseches Pesachim): If a Pesach offering became mixed with Bechoros (firstborn offerings), R. Shimon says that if the Pesach offering belongs to a group of Kohanim, they should eat it (as they are permitted to eat Bechoros). And a Baraisa teaches that they should eat according to the stricter of the two (i.e. the Pesach offering, that may only be eaten until midnight of the first night of Pesach and only by those registered on it). (This shows that R. Shimon permits even when its time is reduced and it is closer to becoming disqualified. This is a question against R. Yona.)

Answer (R. Mana to R. Chunia): Those who eat the Pesach offering in its time are careful like those who eat Terumah.

Proof (Mishnah in Maseches Shabbos): One may only roast meat, onion and egg before Shabbos if they will be semi-roasted before Shabbos. (If not, there is concern that a person might stoke the coals on Shabbos to advance the roasting.)

(Next Mishnah there): One may lower the Pesach offering into the oven immediately before Shabbos. (The permission here shows that people who prepare the Pesach offering are careful and will remind each other not to stoke the coals.)