Gemara

(Rava bar Ahila'i): Rav exempts one who admits to a fine, even if witnesses came later. He learns from "Im Himatzei Simatzei", for "whom the judges will convict" already excludes one who incriminates himself.

74b (Rav Huna, and Shmuel): He is liable after witnesses testify.

(Mishnah): If witnesses testified that Reuven stole, and we know that he sold from one witness or from Reuven's admission, he pays Kefel, but not four or five.

This equates admission and one witness. When one witness testified, if another witness comes later, they join to make him pay. Similarly, when he admitted, if witnesses later come, he is liable! This is unlike Rav Huna.

Question (Rav Chisda - Beraisa): Once, R. Gamliel told Yehoshua 'I blinded my slave's eye.' R. Yehoshua said 'he does not go free, for you have no witnesses.'

Inference: If he had witnesses, the slave would go free, even though R. Gamliel already admitted. This contradicts Rav Huna!

Answer #1 (Rav Huna): This was because he did not admit in front of Beis Din.

Objection: R. Yehoshua was the Av Beis Din!

Answer #2 (Rav Huna): His admission did not count, for it was not in Beis Din.

(Rav Hamnuna): Rav exempts from Kefel when he admitted before witnesses came and obligated himself to pay principal. Admission to selling after testimony about the theft does not obligate him, so if witnesses came later, he is liable.

Rava: Rav Chisda asked from the Beraisa. Rav Huna did not answer that R. Gamliel's admission did not help because he did not obligate himself!

Support (for Rav Hamnuna - Rav Ashi - Mishnah): If witnesses testified that he stole, and one witness testified or he admitted that he sold, he pays only Kefel.

We equate admission to selling and testimony of one witness. If more witness(es) come later, he pays four or five only when his admission did not obligate himself. However, if one witness testified that he stole and sold, this is unlike admission. If one witness came, and later a second comes, he pays four or five. When he admitted and obligated himself to pay principal, he is exempt from the fine.

Rishonim

Rif and Rosh (7:8): Admission exempts from a fine only if he obligated himself.

Rif (Bava Metzia 19a): We don't collect fines in Bavel, but if the victim seized, we do not take it from him, in Eretz Yisrael or Chutz la'Aretz.

Rambam (Hilchos Geneivah 3:8): If one admitted to a fine in front of Beis Din in Beis Din and witnesses came later, he is exempt. If he admitted outside of Beis Din, or in front of two, and witnesses came later, he pays a fine.

Magid Mishneh: The Rambam holds that even admission in front of two judges does not exempt, for fines require three expert judges. Why did the Gemara need to say that R. Yehoshua was not in Beis Din? If he was in Beis Din, surely his colleagues would have been with him.

Rambam (9): If one admitted in Beis Din that he stole, and later, witnesses testified that he stole, he is exempt, for he obligated himself to pay principal. If he denied stealing, and witnesses testified that he stole, and he admitted that he slaughtered or sold, and later witnesses testified that he slaughtered or sold he pays four or five, for he totally exempted himself until witnesses came.

Rosh (1:20): The Ramban says that nowadays we have no Semuchim, so every admission is like admission outside of Beis Din, which does not exempt (84b). Therefore, we do not take from a victim who seized before or after admission. The Ramah says that seizure helps only up to the value of the damage. If one seized more, e.g. Kefel, we remove it. We find that Chachamim allowed keeping only up to the damage. Also, it is an enactment that seizure helps. Chachamim enacted only up to the damage, so the victim will not lose, but not that he should profit from others' money. I say that it is mid'Oraisa law, just no judge in Bavel can enforce it. Therefore, the victim may take the law into his own hands.

Ritva (Shevu'os 33b Sof DH v'Im): If David claimed that Levi raped David's daughter, and Levi said 'I never raped a girl', and witnesses testify that they saw him rape, he is exempt from the fine, for he incriminated himself. Just like admission exempts, 'admission' through denial exempts. The Yerushalmi obligates a fine in this case.

R. Meir Simchah (Shevu'os 34a DH v'Hinei): Rashi says that admission exempts even if witnesses come later because he wanted to return money. One who denied is liable if later witnesses testify that he raped her. The Yerushalmi obligates even through witnesses who do not know whom he raped. It holds that liability to pay a fine comes immediately, if witnesses testify later. We found out through the witnesses and his (denial, which works like an) admission. He is liable because he did not admit before we knew.

Hagahos (in Tur ha'Shalem CM 348:30): Sha'ar ha'Mishpat (1) says that if one accepted judges of Chutz la'Aretz to judge a fine, he is exempt, for he incriminated himself.

Poskim

Rema (CM 348:4): The Mechaber did not teach when a Ganav pays four or five because it is a fine and does not apply nowadays. However, it is relevant if he seized the money; see CM 1:5.

SMA (11): Perhaps the Mechaber holds like the Ramah; it is an enactment that seizure helps up to the value of the damage. The Rema holds like the Rosh, that mid'Oraisa one who seized the full fine need not return it.

Tur (348): A Ganav pays Kefel, four and five only if witnesses came. If he admitted, he pays only principal, even if witnesses came later, for he obligated himself to pay principal. If he denied stealing, and after witnesses testified that he stole he admitted that he sold, and later witnesses testified that he sold, he pays four or five. Admission to a fine exempts only in front of Beis Din and in the place of Beis Din. Admission in front of two, or in front of three outside of Beis Din, does not exempt from a fine.

Beis Yosef (DH u'Mah she'Omar v'Afilu): One is exempt only if he obligated himself and intended to return money.