Gemara

(Mishnah - R. Yehoshua): Me'ilah applies only to things that never had She'as ha'Kosher (a time when they were permitted) to Kohanim. This excludes something that was Lan, Yotzei (left its permitted place to be eaten), or became Tamei.

Suggestion: This refers to proper Linah (Zerikah was not done). There is no Me'ilah because we consider it as if Zerikah was done!

Rejection #1: No, She'as ha'Kosher is if it was Lan at a time when it was able to be Nifsal (only) due to Yotzei or Tum'ah (i.e. after Zerikah). Alternatively, it is as if it was able to be Nifsal due to Linah, Yotzei or Tum'ah (i.e. after Zerikah).

Objection: This implies that if there was proper Linah (i.e. Zerikah was not done), Me'ilah applies. If so, it should not say that Me'ilah applies if and only if the matter (potentially) had She'as ha'Kosher, rather, if it (truly) does not have She'as ha'Kosher!

Rejection #2 (Rav Ashi): Me'ilah depends on Kedushah. Once the Kedushah vanished (because Zerikah may be done, permitting it), it does not return;

(Mishnah - R. Meir): If one brought an Asham Taluy, and found out after Shechitah that he did not transgress (it is Pasul), the blood is spilled, and the meat is burned. If Zerikah was done, the meat may be eaten;

R. Yosi says, even if the blood is still in the Kli, Zerikah is done, the meat is eaten.

(Rava): R. Yosi holds like R. Shimon, who considers blood ready for Zerikah as if it was thrown.

Rejection: This is not R. Yosi's reason! Rather, R. Yosi bar Chanina taught that R. Yosi holds that Klei Shares are Mekadesh Pesulim to be offered l'Chatchilah.

Gitin 20a (Chachamim of Eretz Yisrael): A Get written on Isurei Hana'ah (something one may not benefit from) is valid.

Support (Rav Ashi - Mishnah): One may write a Get on an olive leaf.

Rejection: An olive leaf is not worth a Perutah, but many leaves together are worth a Perutah. Isurei Hana'ah have no value at all!

(Beraisa - Rebbi): A Get written on Isurei Hana'ah is valid.

Levi taught this in Rebbi's name, and the audience was unreceptive. Levi taught this in the name of Chachamim, and the audience was receptive. This shows that this is the Halachah.

Chulin 89a (R. Ze'ira): One may cover the blood with dirt (i.e. ashes) of an Ir ha'Nidachas (a city burned because most of its inhabitants served idolatry.)

(Rava): Even though one may not benefit from the city, using something for a Mitzvah is not considered benefit.

Question (Beraisa): One is not Yotzei with a Shofar or Lulav of idolatry.

Answer (Rav Ashi): There is a Shi'ur (minimal required size) for a Shofar and Lulav. Since idolatry must be burned, Ketusei Michtas Shi'urei (it is not considered to have a Shi'ur). This does not apply to Kisuy ha'Dam. Ashes are fine for covering!

Sukah 35a: An Esrog of idolatry is Pasul, for Ketusei Michtas Shi'urei. An Esrog of Orlah is Pasul, either because one may not eat it, or because it is not considered your property.

Rishonim

Rif (Gitin 10a): The Halachah is, a Get on Isur Hana'ah is valid. (Some texts omit this.)

Ran (DH Shalchu): A Get on any Isur Hana'ah is valid, even if Ketusei Michtas Shi'urei. Ketusei Michtas Shi'urei applies only to something that requires a Shi'ur, e.g. a Shofar or Lulav. However, if an Isur must be eradicated from the world, according to the opinion that anything destined to be burned is considered as if it were already burned, it seems that it is not a Get, for it is as if it does not exist.

Gra (2): This is astounding. R. Shimon holds that anything destined to be burned is considered as if it were already burned. All agree about something that must be eradicated from the world (Sukah 31b, Chulin 89b). A Lechi (a vertical post that permits carrying in an alley) from idolatry is valid, for Ketusei Michtas Shi'urei applies only to something that has a Shi'ur in all three dimensions. (Magid Mishneh Hilchos Eruvin 17:12). The Gemara mentions Ketusei Michtas Shi'urei only regarding something that must be eradicated! This is why in Sukah 35a, the Gemara needed a reason other than Ketusei Michtas Shi'urei to disqualify an Esrog of Orlah, even though the Mitzvah is to burn it. The Meforshim say so, unlike Tosfos. Rather, the text of the Ran should say 'this is for all Isurim, even those that must be eradicated, and even though Ketusei Michtas Shi'urei, (this is a problem only for things such as) Shofar and Lulav. However, according to the opinion that anything destined to be burned, even if there is no required Shi'ur, it is as if the matter is not here. He would disqualify even a Lechi of idolatry.

Rambam (Hilchos Gerushin 4:2): One may write a Get on anything, even on Isurei Hana'ah.

Rambam (Hilchos Pesulei ha'Mukdashim 4:19): If one realized that he did not sin after blood of his Asham Taluy was thrown, we eat the meat.

Rosh (Chulin 7:37): If a Se'ah of Terumah fell into 100 of Chulin, and before he was able to remove a Se'ah from the mixture, another Se'ah of Terumah fell in, Chachamim forbid and R. Shimon permits. The Yerushalmi says that R. Shimon holds that knowledge permits. I.e. if he knew that the first fell before the second fell, since he was destined to remove a Se'ah, it is considered as if it were already removed. Chachamim disagree. From R. Shimon, we infer that regarding other Isurim, which need not be removed, Chachamim agree that knowledge permits.

Tosfos (20b DH bi'Ksoves): Even though it is forbidden to write a Get on Isur Hana'ah, for he benefits from it, it is Kosher.

Poskim

Shulchan Aruch (EH 124:1): A Get written on Isurei Hana'ah is valid.

i. Bach (1): The Tur taught this together with writing on anything detached to teach that it is l'Chatchilah. One might have thought that 'a Get written on Isurei Hana'ah is valid' connotes b'Di'eved. The Tur learns from Rav Ashi. He brought a support from a Mishnah that permits writing a Get on an olive leaf, which is l'Chatchilah. This shows that l'Chatchilah, one may write a Get in Isur Hana'ah l'Chatchilah. Even though his support was rejected, we see that Rav Ashi held that 'a Get written on Isurei Hana'ah is valid' is l'Chatchilah. Even though Tosfos forbids, it seems that the Tur's opinion is primary, like I proved from the Gemara. It is forbidden to benefit from Isur Hana'ah only when one intends to benefit. When he does not intend, even though it is possible (to avoid benefit), it is permitted, like the Gemara concludes in Pesachim (26b). Here he does not intend to benefit from the Isur Hana'ah. He intends only to write a Get.

i. Rebuttal (Chelkas Mechokek 1): This is the ultimate benefit! He uses Isur Hana'ah for his needs, and saves the money he would have needed to buy a parchment! This is no less than one who slaughtered a dangerously sick animal with a knife of idolatry, which is forbidden. There is no greater benefit than expelling an evil wife from his house to cure himself of this plague. Even though the Tur and the Rambam say 'l'Chatchilah', they mean that the Get divorces absolutely. This is like the Rosh wrote at the beginning of Shabbos, that the Mishnah says 'permitted' regarding Shabbos, but it is forbidden (for Ploni to let Almoni, who is standing in another Reshus, put something into Ploni's hand) due to Lifnei Iver.

i. Beis Shmuel (1): The Shulchan Aruch connotes that it is b'Di'eved, like Tosfos. The Tur and the Rambam holds that it is l'Chatchilah.

Rema: However, if an Isur requires Bi'ur (eradication) from the world and must be burned, if one wrote a Get on it, it is Batel.

Chelkas Mechokek (1): Why does the Rema say Stam that it is Batel? The Ran said so only according to the opinion that anything destined to be burned is considered as if it were already burned, i.e. R. Shimon. This is because he holds that anything destined to be thrown is considered as if it were already thrown. The Halachah does not follow him, like the Rambam (Hilchos Pesulei ha'Mukdashim 4:19) says.

Beis Shmuel (2): The Ran said that it is Batel according to the opinion that anything destined to be burned is considered as if it were already burned, for then it is as if the letters float in the air (nothing keeps them in place). There is no proof from Hilchos Pesulei ha'Mukdashim that the Rambam rules unlike R. Shimon. The Chelkas Mechokek overlooked Krisos 24b, which says that R. Shimon says that anything destined to be thrown is considered as if it were already thrown only if indeed it should be thrown, but if one did not sin, the blood should not be thrown! The Kesef Mishneh (3:19) says that the Rambam rules like R. Yosi, who says that a Keli Shares is Mekadesh Pasul blood to be thrown. This is wrong. If so, we should eat the meat even if he found out before the blood was thrown! Rather, it is like I said, that when he finds out that he did not sin, the Asham Taluy is Chulin retroactively. In other cases the Rambam holds that anything destined to be thrown (or burned) is considered as if it were already thrown (or burned). The Rambam (Hilchos Me'ilah 3:1) says that something is considered permitted to Kohanim only after Zerikah. Menachos 102a connotes that R. Shimon would say so even before Zerikah, for it is destined to be thrown. Tosfos (Me'ilah 5a DH Heter) says that the Sugya holds that even R. Shimon agrees that regarding Me'ilah, the Kedushah goes away only after Zerikah. The Rambam in Hilchos Terumos 13:6) connotes that he rules like R. Shimon (regarding Terumah); see the Rosh above.

Note: The Rosh says that from R. Shimon, we infer what Chachamim hold. This connotes that the Halachah follows Chachamim.

Gra (2): The Rema is difficult. He overlooked that a Lechi of idolatry is Kosher. Also, he said this Stam, and it is only like R. Shimon, and in many places the Sugya is unlike R. Shimon. Chachamim argue with him in many places.