Ketubot, Chapter Three, Mishnah Nine
The concept taught in this mishnah is that one who admits of his own accord that he has committed a crime, without before having evidence brought against him, is exempt from paying the fine. He must, however, pay any penalties that are compensatory and not considered fines.
1) He who declares, I seduced the daughter of so-and-so must pay compensation for embarrassment and blemish on his own admission but need not pay the fine.
2) He who declares, I have stolen must make restitution for the principal on his own evidence but need not repay double, fourfold or fivefold.
3) [He who declares,] My ox has killed so-and-so or the ox of so-and-so must make restitution on his own evidence.
a) [If he said] My ox has killed the slave of so-and-so he need not make restitution on his own evidence.
4) This is the general rule: whoever pays more than the actual cost of the damage he has done need not pay it on his own evidence.
Section one: If a person admits that he seduced someones daughter he does not pay the fine. He does, however, make the other payments.
Section two: A thief is liable to pay back double the amount which he stole. If he slaughtered or sold the animal he must pay back four or five times its value. The double, fourfold and fivefold payments are fines, whereas the restitution for the principal is not a fine. Therefore, if a man admits to having stolen something, he only pays the principal.
Section three: If a persons ox kills someone or someone elses ox, the ox owner must make financial restitution (see Exodus 21:30). This is not considered a fine and therefore if a person admits that his ox killed another ox or a human being, he must make restitution. However, if an ox kills a slave there is an automatic penalty of 30 shekels (see Exodus 21:32). Since this is a fixed sum, a person who admits that his ox did so is not liable to pay the fine.
Section four: The mishnah now sums up what we learned above. If a person is liable to pay a fine that is more than the actual damage, or actually a sum that is set arbitrarily and is independent of the damage, he doesnt pay upon his own admission. The reasoning behind this may be that fines are in order to prevent the person from committing another crime. Therefore, if he comes forward and admits to what he has done (and it was not otherwise known) the need for a penalty does not exist. On the other hand compensation is needed for the loss incurred by the victim and therefore compensatory penalties are paid in any case.