Gemara

(Mishnah): If Levi accepted (answered 'Amen' to) an oath that he did not steal Yehudah's ox, and admitted by himself, he pays principal and Chomesh and brings an Asham.

Chulin 51a (Rav Menasheh): If a thief dropped rams over the wall to return them, surely we are concerned lest (he did not drop them gently and) their limbs were crushed.

This is if he returned them out of fear of being caught. If he returned them to repent, we are not concerned. Surely, he wants to repent properly!

Yevamos 47a: A man told R. Yehudah that he converted by himself. R. Yehudah found that he has no witnesses about this, and he has children.

R. Yehudah: You are believed to disqualify yourself, but not to disqualify your children.

Question (Beraisa - R. Yehudah): "Yakir" - he will inform others. This shows that a man is believed to say who is his firstborn. Similarly, a Kohen is believed to say 'my son was born to a divorcee or a Chalutzah (he is a Chalal)';

Chachamim say, he is not believed.

Answer (Rav Nachman bar Yitzchak): R. Yehudah told him 'by your own admission you are a Nochri, so your testimony is invalid.'

Rishonim

Rif (in Teshuvah, cited in Tur CM 87): If Reuven swore Heses, and later admitted by himself to part of the claim, he is suspected (to swear falsely). The claimant swears about the rest and receives it.

Rebuttal (Tur, Sefer ha'Terumos 10:1:3 cited in Beis Yosef DH Kosav): The Rif is astounding. How can one make himself suspected?!

Defense (Bach 43): The Rif did not mean that we do not let him swear because he is disqualified. Rather, since one is believed to obligate himself through his admission, due to his admission he is suspected, so the claimant swears and receives if he wants. However, if the claimant says 'Reuven must swear to me mid'Oraisa, for he admits to part', we let Reuven swear. He is not disqualified through himself. The Tur wrote so in Siman 92 in the name of the Ramah, without any dissenting opinion.

Rav Shrirah Gaon (cited in Tur, ibid.): If he admitted by himself, if he was like one who repents and regrets that he swore falsely, he is exempt from the rest. If he did not admit like one who repents, he swears about the rest. He is not disqualified from swearing through his admission.

Beis Yosef (ibid.): R. Yerucham (3:2:16b) brings only Rav Shrirah Gaon. 'He swears about the rest' is difficult. We are forced to explain 'the oath he already took exempts him from the rest.'

Rebuttal (Bach ibid.): If so, there is no distinction between admission like Teshuvah or not! Rather, Rav Shrirah Gaon means that he swears now about the rest. Perhaps he agrees with the Rif. He says that Reuven swears about the rest when the claimant does not want to swear. This is like Sa'if 23, in which after Reuven swore Heses, the claimant brought one witness. The Rif says that Reuven now swears mid'Oraisa.

Rambam (Hilchos To'en 2:3): One does not become suspected until witnesses come and say that he transgressed an Aveirah for which one is disqualified. If one admits that he is suspected and transgressed, even though we suspect him and it is improper to make him a witness, if he must swear to exempt himself, we make him swear. We tell him 'if you say the truth, swear! You are Kosher to swear. If you are lying, admit!

Ran (Shevuos 28b DH Mishnah, citing the Rashba): How can we know that one transgressed Shevu'as ha'Edus b'Mezid? Perhaps he forgot!

Answer (Ran): If he swore very soon after seeing it, we are not concerned lest he forgot. Also, he is believed to say that he was Mezid. One cannot disqualify himself from another testimony. One who says that he is suspected (about oaths) obligates himself to pay through his admission, like 100 witnesses.

Tosfos Yom Tov (Shevuos 7:4 DH Shevu'as): This is only if the claimant agrees to swear and collect. If not, he can say 'swear to exempt yourself, or pay.' If not, it would be advantageous to disqualify oneself, for we do not reverse a mid'Oraisa Shevu'ah! This is like the Ramah. The Rambam holds that one cannot disqualify himself.

R. Akiva Eiger: Perhaps the Rambam agrees with the Ran. The Rambam says that he swears when the claimant does not want to swear, but the claimant may opt to swear and collect! The Tur brings the Rambam and the Ran, and connotes that they do not argue.

Tosfos (Yevamos 47b DH v'Ein): R. Yehudah believed the man to disqualify himself regarding Shavya a'Nafshei Chatichah d'Isura, i.e. by his admission this is forbidden to him, or to obligate himself monetarily.

Poskim

Shulchan Aruch (CM 87:28): If Reuven swore Heses, and later admitted by himself to part of the claim, if he was like one who repents and regrets that he swore falsely, he is exempt from the rest.

Gra (98): We learn from Chulin that one who repents, repents properly.

Shulchan Aruch (ibid): If he did not admit like one who repents, he is not disqualified from swearing through his admission.

SMA (88 and Drishah 43): When he admits like one who repents, we say that if he owed more, he would have admitted to it. Alternatively, the claimant would not have claimed any more, so Reuven is like one who returns an Aveidah. Therefore, he need not swear again. When his admission was not like one who repents, he must swear again, for his first oath was false. The Shulchan Aruch did not mention swearing again. The Beis Yosef explains that in any case, he is exempt. If he admitted like one who repents, if he owed more, he would have admitted to it. If he admitted unlike one who repents, in any case his first oath was valid, for one cannot disqualify himself.

Gra (99): One cannot disqualify himself from testimony (Sanhedrin 9b). Likewise, one cannot disqualify himself from swearing. To obligate oneself, he can disqualify himself.

Shach (72): If the claimant wants, he swears and collects, against Reuven's will.

Shulchan Aruch (92:5): One does not become suspected until witnesses come and say that he transgressed an Aveirah for which one is disqualified. If one admits that he is suspected and transgressed, even though we suspect him and it is improper to make him a witness, if he must swear to exempt himself, we make him swear. However, if the Halachah is that he swears and collects, he does not do so.

Shach (5): L'Chatchilah, we do not ask him to witness an event. Obviously, if he knows testimony he is Kosher!

Shulchan Aruch (ibid): Some say that if he must swear to be exempt, since he says that he is suspected, the claimant may opt to swear and collect.

Beis Yosef (DH u'Mah): The Tur says so in the name of the Ramah. Why may the claimant swear Bal Korcho (against his will)? It is because according to himself, he is suspected.

SMA (18): If the claimant does not want to swear, he can say 'you cannot exempt yourself from swearing Heses, or reverse a Shevu'ah mid'Oraisa onto me. Swear or pay!'

Shach (6): This refers to a Shevu'ah mid'Oraisa. Regarding Heses, he says 'if you believe me totally (that I am suspected), believe me without swearing that I am exempt;, for we do not make an enactment (to reverse the oath to the claimant) for an enactment (Shevu'as Heses).