Gemara

(Beraisa) Suggestion: If a Kohen redeemed a field that will be given to Kohanim in Yovel, he cannot say "since it is given to Kohanim, and I have it now, I will keep it. A Kal va'Chomer supports me!

I acquire (a share of) things I had no rights to, e.g. a field that a Yisrael redeemed. All the more so, I acquire this field, which I already have!"

Rejection: "Achuzaso" teaches that a Kohen keeps his Achuzah, but not this.

Makos 11b (R. Ami or R. Yitzchak Nafcha): If a Shogeg murderer was sentenced, and then the Kohen Gadol was found to be a Chalal (his mother was forbidden to Kohanim, e.g. a divorcee), it is as if the Kohen died (the murderer is free);

(The other of R. Ami and R. Yitzchak Nafcha): Kehunah was Batel. (It is as if he never was Kohen Gadol. The murderer never leaves the Ir Miklat.)

Suggestion: The first opinion holds like R. Yehoshua, and the second opinion holds like R. Eliezer:

(Mishnah - R. Eliezer): If a Kohen was offering Korbanos, and he was found to be a Chalal, the Korbanos are Pasul;

R. Yehoshua says, they are valid.

Rejection: The second opinion could hold like R. Yehoshua;

R. Yehoshua learns from "Barech Hash-m Cheilo u'Fo'al Yadav Tirtzeh" that the Avodah of Chalalim is acceptable (b'Di'eved), but in other respects, he is not a Kohen.

Pesachim 64b (Mishnah - R. Eliezer): If a Kohen was eating Terumah, and found out that his mother was divorced, he pays Keren v'Chomesh;

R. Yehoshua exempts.

(Rav Bibi Bar Abaye): The case is, it was (Chametz on) Erev Pesach. There is limited time to eat it. (This is why R. Yehoshua exempts.)

Alternatively, eating Terumah is called Avodah. B'Di'eved, (R. Yehoshua holds that) Avodah of a Chalal is Kosher.

Kidushin 66b (Beraisa - R. Tarfon): If a Kohen was offering Korbanos, and it became known that he was born to a divorcee, what he offered is acceptable.

(Rav Yehudah): We learn from "(Kehunah) will be to him (Aharon) and his seed", whether Kosher or disqualified seed;

(Shmuel's father): We learn from Barech Hash-m Cheilo...

(R. Yanai): We learn from "you will come to the Kohen who will be in those days";

This refers to a Kohen who was Kosher, and then was found to be a Chalal.

(Beraisa - R. Akiva): If a Kohen was offering Korbanos, and it became known that he was blemished, what he offered is not acceptable.

(Rav Yehudah): We learn from "I (Hash-m) give to (Pinchas) My covenant of Shalom." This is only when he is Shalem, but not when he is lacking.

(Beraisa - R. Tarfon): If a Mikveh was Muchzak to have the required amount of water, and it was measured and found to be deficient, what is the law of what was immersed in the Mikveh? Is this like a Kohen found to be a Chalal, or like one found to be blemished?

Bechoros 47b (Rav Chisda): If a Kohen fathered a Chalal, and died, the son (if he is a Peter Rechem) must redeem himself.

(Rabah bar Rav Huna): He need not redeem himself.

If the father died after 30 days, all agree that the son is exempt. It is as if the father redeemed him and kept the money himself.

Rabah exempts even if he died within 30 days. Since (the son's obligation comes from his father's, and) Kohanim could not have forced his father to pay, also he is exempt.

Rishonim

Rambam (Hilchos Rotze'ach 7:12): If a final verdict was given (for Galus), and then the Kohen Gadol was found to be a Chalal, the Kehunah is Batel. It is as if he was sentenced without a Kohen Gadol. He never leaves Galus.

Kesef Mishneh: The Rambam rules like the opinion that Kehunah died due to Kidushin 66b, in which we say that R. Yehoshua expounds Barech Hash-m Cheilo, which is like the opinion that Kehunah was Batel. Also, the Yerushalmi holds like this.

Rebuttal (Yad Eliyahu 37): The Rambam rules that Kehunah was Batel because it is a Safek mid'Oraisa. Also, if the Go'el ha'Dam would kill the murderer outside the Ir Miklat, we could not judge him. Therefore, he rules that he must stay in Galus.

Rambam (Hilchos Terumos 10:12): If a Kohen was eating Terumah, and found out that his mother was divorced, he pays Keren v'Chomesh. If it was Chametz on Erev Pesach, he is exempt, for he was frantic to eat it and he was not able to check.

Poskim

Shulchan Aruch (YD 305:3): One must give the five Sela'im for Pidyon ha'Ben to a Kohen.

Question: A case occurred in which Ploni came and established himself to be a Kohen. Years later, Reuven came and swore that Ploni is not a Kohen, and Ploni admitted. What is the law of Pidyon ha'Ben that Ploni received?

Answer (Chavos Ya'ir 113, cited in Pischei Teshuvah 5): Had Ploni insisted that he is a Kohen, one witness would not be believed to change the Chazakah. I say that we are concerned that he desires to marry a divorcee or Chalalah, and schemes to permit himself. We apply to him stringencies of Kohanim and Yisre'elim. All Bechoros redeemed through him must be redeemed again. Even though we hold like R. Yehoshua, that if a Kohen was found to be a Chalal, the Korbanos he offered are Kosher, there is different, for we learn from a verse. Regarding other matters, we hold like the opinion that Kehunah is Batel.

Rebuttal (Tad Eliyahu 37): This is astounding. Surely, no one is Machshir the Avodah of a Zar b'Di'eved!

Note: Perhaps Chavos Ya'ir teaches that there is no Safek. I.e. do not say that witnesses may call a Chalal a Yisrael, and perhaps the Pidyon was valid b'Di'eved.

Suggestion: We learn from a Kal va'Chomer. Korbanos are severe; even a Mum disqualifies them. Even so, they are Kosher (if the Kohen was found to be a Chalal). All the more so, other Matanos Kehunah (are valid); the Bechoros need not be redeemed again! (Do not say that if this is a Kal va'Chomer, it should be Machshir even if he was found to be a Ba'al Mum. A verse teaches that Avodah of a Ba'al Mum is Pasul. This overrides the Kal va'Chomer.)

Rejection (Chavos Ya'ir): We hold that if a Kohen ate Terumah and he was found to be a Chalal, he pays a Chomesh, like a Zar who ate Terumah. He is exempt only on Erev Pesach, like the Rambam says. Surely he must return all money received for Pidyon ha'Ben.

Tumim (38:1, page 85b middle of column 2 in standard editions): The Rambam holds that 'like they plotted, and not like they did" applies only to capital cases. I prove this from Makos 2a, which says that if Edim Zomemim testified that Shimon is not a Kohen, they are lashed. There is no liability before their testimony is accepted, and once it is accepted, this is 'like they did' (the matter is finished)! Do not say that they testified that Shimon accepted money for Pidyon ha'Ben, and he must return it. If so, they owe money, and they are not lashed.

Question: If a Kohen received Pidyon ha'Ben, and he was found to be a Chalal, must he return the money? If he need not, must the Bechor redeem himself again when he matures? What is the Tumim's source to say that he must return it? R. Yehoshua exempts a Kohen who ate Terumah and was found to be a Chalal, for this is called Avodah. Perhaps the same applies to Pidyon ha'Ben, for also it is called Avodah!

Answer (Be'er Sheva YD 25): Eating Terumah is called Avodah, for it is a Mitzvah for the Kohen to eat it. We do not find that it is a Mitzvah for a Kohen to receive five Sela'im. Surely he must return the money, like the Tumim says. A proof is from Bechoros 47b. A Chalal must redeem himself. The Rambam is Machshir Avodah of a Chalal, even if he served after it was known that he is a Chalal. Even if we discuss Pidyon after it was known that he is a Chalal, if b'Di'eved a Chalal need not return the money, since if others would give to him, he need not return it, all the more so he acquires his own. We say so in Erchin. Here, he holds the money to redeem himself! This is even according to those who say, unlike the Pri Chodosh, that a minor Kohen cannot receive Pidyon ha'Ben. Since when he matures he can receive and need not return it, all the more so he acquires his own.

Be'er Sheva: Do not say that in Bechoros we require him to redeem himself only l'Chatchilah. If b'Di'eved he could keep the money, Beis Din could not force him. 'He is liable' connotes that Beis Din forces him. One opinion is Machshir Avodas Chalal due to "you will come to the Kohen..." The Yerushalmi says that this opinion is Machshir him even for matters other than Avodah, e.g. to permit a murderer to leave his Ir Miklat and ruling about Tzara'as. The Gemara said that Rabah exempts because Kohanim could not have forced his father to pay. Why didn't it say that they argue about which verse is Machshir Avodas Chalal?! The one who learns from Barech Hash-m Cheilo is Machshir only for what is called Avodah! What forced the Gemara to say that it is a new argument? Rather, all agree that he is Kosher only for Avodah. Also regarding Terumah, R. Yehoshua exempts only from the Chomesh. He agrees that the Kohen must pay the principal, like the Kesef Mishneh explains. The Yerushalmi supports this.

Question: A man established himself to be a Kohen, and received Pidyon ha'Ben for many Bechoros. Witnesses testified that he is a Yisrael, and he admitted. Do we say that since he was Muchzak, and we rely on Chazakah to stone people, the children are redeemed? Or must they redeem themselves, and also he must return the money?

Yad Eliyahu (37): Since now there is clarity, the 'Chazakah' was shown to be false. Surely he must return the money, and the Bechoros must be redeemed. If it is a Safek whether the money was given to him or to a (real) Kohen, ha'Motzi mi'Chavero Alav ha'Re'ayah. Do not say that we say so only for a Safek Bechor, but here the Bechor was surely obligated (before 30 days), so Chazakah says that he is still liable. We do not force people to pay even due to a majority, which even stronger than Chazakah. Presumably, there were more Kosher Kohanim than him (so most Bechoros were properly redeemed). If he was the only Kohen in the city, surely we assume that Bechoros were redeemed through him.

Yad Eliyahu: If it became known that he is a Chalal, it is not clear whether he must return the money. We hold like R. Yehoshua. This is not only for Avodah. The Gemara brings this to learn to other laws, e.g. the Pesul of a Mikveh and Galus. The Gemara did not settle the latter law. It is just like Pidyon ha'Ben. It seems that we favor the opinion that Kehunah died, for it says even R. Yehoshua could admit here (that Kehunah was Batel), since he learned from Barech Hash-m Cheilo. This implies that according to the other two Drashos (to be Machshir Avodas Chalal), he would say that Kehunah died. Even if the one who expounds Barech Hash-m Cheilo holds that only Avodah was Kosher, we hold like the other two against the one. Also the Yerushalmi holds like the opinion that Kehunah died. I say that the Rambam rules that he must stay in Galus because it is a Safek Torah. Regarding Pidyon ha'Ben, ha'Motzi mi'Chavero Alav ha'Re'ayah. The Kohen need not return the money. The Gemara answered that he is exempt for Terumah if it was Erev Pesach. Tosfos (Yevamos 34a DH b'Erev) says that it could also have answered that eating Terumah is Avodah.