Gemara

31b (Beraisa): Ma'aser Behemah of orphans may be sold. (The meat of) a slaughtered Ma'aser Behemah may be sold b'Havla'ah (together, for one price) with the skin, Chelev, sinews and horns.

Question: What does this mean?

Answer #1 (Abaye): A (slaughtered) Ma'aser Behemah of orphans may be sold b'Havla'ah.

Question: This implies that Ma'aser Behemah of adults may not be sold (even) b'Havla'ah. Why is this different than an Esrog of Shemitah, which may be sold b'Havla'ah?

(Mishnah): When one buys a Lulav from an Am ha'Aretz in Shemitah, the seller gives an Esrog for a gift, for one may not buy it.

(Rav Huna): If the seller does not agree to give it for a gift, one buys it b'Havla'ah with the Lulav.

Answer: Regarding an Esrog, the ruse is not evident, for the price charged for the Lulav is reasonable. Regarding Ma'aser, the ruse is evident, for the meat is worth much more. No one would pay so much for skin alone.

Objection (Rava): If the Beraisa were all one clause, it would not say "Ma'aser Behemah"... and then repeat "Ma'aser Behemah"!

Answer #2 (Rava): Ma'aser Behemah of orphans may be sold normally. A slaughtered Ma'aser of an adult may be sold b'Havla'ah with the skin.

(Rava): I learn from "v'Hayah Hu u'Semuraso Yihyeh Kodesh Lo Yiga'el" - just like Temurah applies while it is alive, also the Isur to sell it. Chachamim decreed against selling it after Shechitah, lest one sell it before Shechitah. They decreed only about what is (important and) commonly sold in the animal's lifetime (i.e. meat). They did not decree about orphans.

Rav Shmuel bar Rav Yitzchak holds like Rava.

(Rav Shmuel): Ma'aser Behemah of orphans may be sold normally. We learn from "Rak b'Chol Avas Nafshecha Tizbach v'Achalta Vasar (ch'Birkas Hash-m Elokecha)." It refers to something that is blessed (i.e. may be sold) only after Shechitah, i.e. Ma'aser Behemah.

Zevachim 9a - Question (R. Yitzchak b'Rebbi Savrin): If a Korban Pesach was slaughtered l'Shem Ma'aser, it should become Ma'aser!

This would cause one who sells to be lashed for "Lo Yiga'el".

Answer: "Ha'Asiri Yihyeh Kodesh" - the only way an animal can become Ma'aser is if it is the tenth (to leave the pen).

Rishonim

Rambam (Hilchos Bechoros 6:5): One may not sell Ma'aser Behemah when it is Tamim (unblemished) - Lo Yiga'el. This includes an Isur to sell it. It may not be redeemed or sold at all.

Ri Korkus: The Rambam forbids from the Torah only when it is alive and Tam. The Gemara connotes that the Torah forbids when it is alive, even if it has a Mum. The Isur to sell a slaughtered Ma'aser (that had a Mum) is a decree lest one sell it alive. Also, Rava expounds that the Isur to sell it is when it is alive, just like Temurah. Temurah applies to a Ba'al Mum! Rav Shmuel learns from "Tizbach... ch'Birkas", something that is blessed only after Shechitah. Rashi explains that other Korbanos may be sold with a Mum while alive. Perhaps the Rambam said 'Tamim' due to the Isur to redeem, which applies only to a Tam. A Ba'al Mum is automatically Chulin! Alternatively, the text should say 'Chai (alive)' in place of Tamim. According to Rava, one may sell it alive if it had a Mum before it was Hukdash, for such a Korban does not make Temurah. Perhaps even Rav Shmuel could agree, i.e. when it was Hukdash Tam, the Berachah is only after Shechitah.

Kesef Mishneh: The Bartenura says that one may not sell it alive even with a Mum. The Mishnah mentioned Tam, for it also discusses a Bechor, which may be sold alive and Tam. From Rava we learn that the Torah forbids only when it is alive and Tam, like Temurah.

Minchas Chinuch (361 DH v'Din): In the next Halachah, the Rambam says that mid'Rabanan, one may not sell Ma'aser Behemah with a Mum, even if it was slaughtered. This shows that the Torah permits selling a live Ba'al Mum. What is the source? The Rambam forbids selling a Tam Stam, i.e. even after Shechitah. In Halachah 7 he is lenient for orphans only regarding a slaughtered Ba'al Mum. This is like Rav Shmuel, who says that the Heter is only for a Ba'al Mum after Shechitah. He rules like him, because Zevachim 9a is like him. Why does the Rambam exempt for a live Ba'al Mum? Rava and Rav Shmuel both obligate for this! The Kesef Mishneh is difficult. Temurah applies also to a Ba'al Mum!

Kiryas Sefer: The Isur to redeem or sell Ma'aser Behemah is when it is Tam - "Yihyeh Kodesh Lo Yiga'el." It is only when it is alive, for then it can make Temurah.

Tosfos (9a DH l'Mai): How can we say that if a Pesach was slaughtered l'Shem Ma'aser, it should become Ma'aser, and one who sells it should be lashed for "Lo Yiga'el"? In Bechoros we say that selling Ma'aser after Shechitah is only mid'Rabanan!

Minchas Chinuch (361 DH v'Nir'eh): Rav Shmuel does not expound Rava's Hekesh. His only Torah source to permit selling after Shechitah applies only to a Ba'al Mum. Zevachim 9a holds like him. However, Rava is Basra, and Rav Ashi holds like him

Ohr Some'ach: The Gemara in Bechoros explicitly says that the Torah forbids selling it when it is alive, even a Ba'al Mum. After Shechitah the Torah permits. Tosfos asked that in Zevachim, we said that if a Korban Pesach was slaughtered l'Shem Ma'aser, it should become Ma'aser, and one who sells it would be lashed for "Lo Yiga'el". Indeed, Rava holds that the Torah forbids even a Ba'al Mum when alive (unlike the Kesef Mishneh). However, Rav Shmuel expounds "Tizbach...", which discusses a Ba'al Mum. There is no Heter to sell a Tam after Shechitah. Even though the Gemara said that Rav Shmuel holds like Rava, he agrees only regarding the Torah's Heter for a slaughtered Ba'al Mum, but not regarding a slaughtered Tam. Also, since the Torah explicitly permits selling, Chachamim would not totally forbid, therefore, they permitted through Havla'ah. However, it seems that Tosfos' opinion is primary. The Gemara learned from the Isur to sell Cherem. The simple meaning of Lo Yiga'el remains, i.e. we discuss a Ba'al Mum, and it teaches that one may not redeem it. We must say that the Isur to sell a Tam is from a Kal va'Chomer. Actually, it is a Giluy Milsa (reveals that the Isur applies also to a Tam), so one is lashed for it. Since the Torah permits selling a Ba'al Mum after Shechitah, and we learn the Isur to sell a Tam from a Ba'al Mum, there is no source to forbid a Tam after Shechitah even according to Rav Shmuel.

Rambam (6): Mid'Rabanan, one may not sell Ma'aser Behemah with a Mum, even if it was slaughtered. This is a decree lest one sell it when alive. Therefore, one may not weigh a portion of Ma'aser against another portion, the way we weigh Bechor, for it looks like selling.

Ohr Some'ach (DH v'Yir'eh): The Tosefta (Kidushin 4:6) says that if one was Mekadesh with meat of Ma'aser, she is not Mekudeshes. Before Shechitah, this is Torah law. After Shechitah, Chachamim uprooted the Kidushin.

Chasdei David (on the Tosefta): The Tosefta discusses an animal bought with money of Ma'aser Sheni.

Rambam (7): If Ma'aser Behemah of orphans had a Mum and it was slaughtered, it may be sold normally, for this is like Hasavas Aveidah (saving them from a loss). Chachamim did not decree against this.

Rambam (8): If Ma'aser Behemah with a Mum was slaughtered, one may sell the Chelev, sinews, skin and bones. Only the meat may not be sold. If one included the money for the meat with the money for the skin, Chelev and sinews, and sold it all together, it is permitted.