Kesubos, Chapter 3, Mishnah 9. The Mishnah opens with the law of someone who admits that he seduced a young unmarried woman: "Ha'omer, pitisi es bito shel ploni - meshalem boshes u'fgam al pi atzmo, v'eino meshalem k'nas" - one who says, "I seduced so-and-so's daughter," pays for humiliation and blemish on the basis of his own admission, but does not pay the fine. Someone who admits on his own that he seduced another man's daughter must pay for the damages of humiliation and of blemish (the reduction in her worth) on the strength of his own admission, but he does not pay the fine that the Torah set - fifty shekel.
The principle of "Modeh Bik'nas Patur":
The basis of this law is the rule that one who admits on his own that he committed an act which carries a fixed fine is exempt from paying that fine. This is derived from the wording of the verse "asher yarshi'un Elokim" - the court is what convicts the liable party, and the verse comes to exclude one who convicts himself through his own admission, in every case involving a fine.
"I stole, slaughtered and sold":
The Mishnah continues with further examples of this principle, in the case of one who admits that he stole a sheep or an ox, and beyond that slaughtered or sold it. For theft the Torah established two fines:
Double payment - one who steals an object pays twice its value, and the additional amount is a fine.
Fourfold and fivefold payment - one who slaughters or sells the stolen animal pays four or five times its value, depending on whether it was a sheep or an ox, and these too are fines.
Therefore the Mishnah says: one who admits that he stole and even slaughtered or sold - "meshalem es hakeren al pi atzmo, v'eino meshalem tashlumei kefel v'sashlumei arba'ah v'chamishah" - he pays the principal on the basis of his own admission, but he does not pay double payment or fourfold and fivefold payment. The principal, meaning the value of the animal itself, he must pay based on his admission; but the fixed fines he does not pay on the basis of his own admission.
"My ox killed so-and-so":
The Mishnah continues: "Hemis shori es ploni o shoro shel ploni - harei zeh meshalem al pi atzmo" - if one says, "My ox killed so-and-so," or "My ox killed so-and-so's ox," he pays on the basis of his own admission. One who admits that his ox killed a person, or killed someone else's ox, must pay based on his admission. In the case where the ox killed another animal, all agree that this is not a fine but actual monetary damage. However, when the ox killed a person, the Torah calls the payment "kofer," and there is a dispute whether kofer is monetary, that is, a payment for damage, or a punishment. Our Mishnah holds that kofer is a payment for monetary damage, and therefore one who admits is liable on the basis of his admission; for if it were a punishment and a fine, he would be exempt.
"My ox killed so-and-so's slave":
In contrast, the Mishnah says: "Hemis shori avdo shel ploni - eino meshalem al pi atzmo" - if one says, "My ox killed so-and-so's slave," he does not pay on the basis of his own admission, and in this case all agree. This is because the Torah set a fixed sum of thirty shekel for one whose ox killed his fellow's slave, with no connection whatsoever to the actual value of the slave. Since this is a fixed sum, it is a fine and a punishment, and therefore it is not paid on the basis of one's own admission.
"This is the rule":
The Mishnah sums up and gives the rule for all these cases: "Kol hameshalem yoser al mah shehizik - eino meshalem al pi atzmo" - whoever pays more than the damage he caused does not pay on the basis of his own admission. Any payment that exceeds the measure of the damage caused is a fine, and by virtue of the principle that one who admits to a fine is exempt, a person does not become liable for it through his own admission.