This question really requires further thought, but I am going to make a very brief comment for the moment.

The Shach (Choshen Mishpat 388:51) writes that Modeh b'Knas Patur is stated only concerning a Knas d'Oraisa, not a Knas d'Rabanan.

It seems to me that we may suggest a proof for the Shach's position from our Gemara. The Gemara asked what the Nafka Minah is whether Rebbi Meir's din is a Knas. If Modeh b'Knas Patur applies also to a Knas d'Rabanan, then the Gemara could have said that there is a Nafka Minah for Modeh b'Knas Patur. The fact that the Gemara did not say this is a support for the Shach's Shitah.

After looking into the matter a little more, I found that the question of whether Modeh b'Knas Patur applies only to a Knas d'Oraisa or even to a Knas d'Rabanan seems to be the subject of a dispute among the Rishonim.

1) The Shach that I cited above is based on the Mordechai in Bava Kama, end of #180. The Mordechai discusses the topic of Moser, one who hands over the property of a Yisrael to a Nochri. The Mordechai writes that if the culprit admits how much loss he caused to his victims, he must pay everything he admitted. Even though Modeh b'Knas is Patur, and even though Dina d'Garmi (an indirect cause of damage, the category in which the Mordechai evidently maintains that Moser falls) is a Knas, nevertheless Modeh b'Knas Patur applies only to a Knas d'Oraisa, while Dina d'Garmi is only Chayav mid'Rabanan.

2) However, the Ketzos ha'Choshen (388:11) cites the Shitah Mekubetzes in Bava Kama (91b, DH Ana Ba'ina) who quotes Rabeinu Yehonasan who maintains that Modeh b'Knas Patur applies even to a Knas d'Rabanan. The Gemara there discusses the case of a person who "stole" a Mitzvah from his friend. He must pay 10 gold coins for doing so. It seems that it is not clear whether this payment is considered a Knas or a Takanah. Rabeinu Yehonasan writes that if is considered a Knas, then Modeh b'Knas Patur applies in such a case. Clearly, the Din that one pays 10 gold coins for stealing a Mitzvah from somebody else, is a Din d'Rabanan. We see from this that Rabeinu Yehonasan maintains that Modeh b'Knas d'Rabanan is also Patur, unlike the Mordechai.

3) Now to answer your question: One way of answering would be to say that according to the Mordechai, the Gazlan is liable even if he admits it, while according to Rabeinu Yehonasan he is exempt.

4) However, another way of answering would be to assume that the Diyuk I suggested in my original brief answer is a good one -- namely, that the fact that the Gemara here did not give the Nafka Minah -- between whether Rebbi Meir maintains that Shinuy b'Mekomo Omed or whether he holds that here it is a Knas -- implies that if it is a Knas, then Modeh b'Knas is Patur. This Diyuk would suggest that everyone here agrees that Modeh b'Knas is not Patur. In fact, I have not found that any of the Mefarshim suggest that this Diyuk in the Gemara is a proof for the Shach in the name of the Mordechai. Possibly, the reason is that in the case of our Gemara, even Rabeinu Yehonasan would agree that Modeh b'Knas d'Rabanan is liable. This may be because the Knas mentioned in our Gemara is different from the standard Knas, since the Gazlan has managed to find a loophole in the law in order to enable him to keep the sheared wool or the babies of the sheep through the concept of "Shinuy Koneh." Therefore, Chazal made a Knas to ensure that the Gazlan should not profit from his crime, and they said that he must repay the wool or the babies anyway. If we would apply Modeh b'Knas Patur in this case, then this would mean that the Gazlan has found yet another loophole to enable him to gain from his crime. Therefore, Rabeinu Yehonasan will agree that even though, generally speaking, Modeh b'Knas is Patur for a Knas d'Rabanan as well, our Gemara is different because the aim of the Knas is to prevent the thief from gaining.

I am still not sure that I have gotten to the bottom of this important question, but I must close here.

Dovid Bloom

I posed your question to a big Talmid Chacham. He commented that it is a very good question, but when he thought about it a little more he declared that the question does not even start, for the following reason. What is the thief admitting to? He is not really admitting to the Knas. Rather, he is admitting that he stole the sheep. However, the Knas is not actually for stealing the sheep, but rather for what happened after the sheep was stolen -- namely, wool grew and was shorn, or a baby was born. According to the Gemara's possibility that this is a Knas, this means that even though mid'Oraisa "Shinuy Koneh," nevertheless Chazal imposed a fine so that the thief should not keep the wool or the baby. When the thief was "Modeh," this means merely that he admitted that he stole it, while the Knas is on a different point: to say that he will not benefit from the future developments of wool or birth. It is not possible to make an admission about the wool or baby because this happened on its own, after the crime of stealing the sheep that he committed.

Kol Tuv,

Dovid Bloom

Sam, I was referred to a source which supports your argument that the reasoning behind Modeh b'Knas is to encourage the thief to do Teshuvah.

1) This is from the Gemara above (75a) where Rav Hamnuna stated that, according to Rav, if the thief said, "I stole," and then witnesses came and also said that he stole, he is exempt from paying the double fine, because his admission did at least obligate him to pay the face value of the theft. However, if he said, "I did not steal," and witnesses came and said he did steal, and then he said, "I slaughtered [or sold] the animal," and witnesses came and also said that he had slaughtered or sold the animal, in such a case we do not say Modeh b'Knas Patur. This is because his admission made him totally exempt from paying anything. Rashi (DH she'Harei) writes that the reason he admitted to the slaughtering is that he knows that Modeh b'Knas is Patur. An admission in such circumstances is not considered an admission. However, in the first scenario, where he did subject himself to paying the Keren through his admission, it is considered that he has "admitted" because "Miskaven l'Hachzir Mamon" -- he genuinely intended to return money.

2) The Nachalas Moshe there (page 147, DH Amar Rav Hamnuna) writes that according to Rav, the reason why Modeh b'Knas Patur is that his admission proves to us that he wants to do Teshuvah, so he no longer deserves a fine. However, if his admission exempted him from any payment, we have no evidence that the thief wants to do Teshuvah (as perhaps he admitted only to exempt himself from some part of the payment), so he still receives the fine.

3) However, if we apply this idea to our Gemara (95a), it may be that he would not pay a fine. He stole a cow and then the cow gave birth. Since he admitted that he stole the cow, this would suggest that he does want to return money, so he could be exempted from returning the baby because we see he has done Teshuvah. Therefore, we may still have to rely on the other answer I gave above -- that he did not actually admit to the Knas since the Knas came only later, when the calf was born.

4) I saw that Rav Dessler writes (in Michtav me'Eliyahu 3:297, DH ha'Torah) that the purpose of the fines of the Torah is to teach us the magnitude of a sin. This is why the fines of the Torah are different from any other fines in the world, in that one can be exempt by admitting to the Aveirah. Once a person acknowledges that he has done wrong, it is no longer necessary for him to suffer the fine.

5) This, then, is a very suitable Sugya for us to be learning in Elul. The main thing is for us first to recognize what shortcomings we have, and everything will follow from there, to help us improve ourselves in the coming year.

Sam, as we approach the New Year, it is an appropriate opportunity for me to express my great thanks to you for always sharing your very thoughtful comments with the Kollel, as you have done for many years now.

Kesivah v'Chasimah Tovah,

Dovid Bloom