בבא קמא עד:
"מודה בקנס ואח"כ באו עדים פטור"
Rav Huna says in the name of רב, if one is מודה that he owes a קנס and afterwards עדים come to ב"ד and say he is חייב the קנס, he is still פטור. The Gemara asks on this from a story where ר"ג blinded the eye of his slave טבי. This made ר"ג very happy, because טבי, although a עבד כנעני, was a righteous talmid chacham and he may now be set free. He then met ר' יהושע and told him, did you hear? My עבד has gone out לחירות. R’ Yehoshua asked him why? ר"ג answered because I blinded him. ר' יהושע responded that this is not the halacha because טבי does not have any עדים that this happened and since you are a מודה בקנס (a slave going out free from being blinded is a קנס), you are פטור from setting him free. The Gemara says we see from the words of ר' יהושע that if טבי did bring witnesses even after ר"ג was מודה, the עדים would be מחייב ר"ג, which is against the halacha of רב. The גמ' says that "מודה בקנס פטור" even if subsequently witnesses come to ב"ד, is a גזה"כ from the pasuk, "אשר ירשיעון אלוקים". We learn: "פרט למרשיע את עצמו". There is a מחלוקת הפוסקים, some hold that this לימוד only says that ב"ד doesn’t have the כח to be מחייב a מודה בקנס to pay the fine, but his חיוב is still there and he is חייב to pay to be יוצא בידי שמים.
The רשב"א [ד"ה אמר] is מדייק from the story of ר"ג that when one is מודה בקנס and is פטור, there isn’t even a חיוב לצאת בידי שמים to pay the קנס. Because if there was a חיוב, then ר"ג would be חייב to free טבי anyway, even though he was a מודה בקנס. It is interesting to note, that the Mishnah [ברכות טז:] relates that when טבי died, ר"ג sat shiva for him. His talmidim asked, didn’t you teach us: "שאין מקבלין תנחומין על העבדים"? To which רשב"ג replied that טבי was different as he was a כשר. We see from this story that ר"ג never ended up freeing טבי.
The גמ' says [עה.] that according to רב who holds "מודה בקנס ואח"כ באו עדים פטור", this is only when the גנב came to ב"ד and admitted that he stole, thereby making himself חייב for the קרן. Only in this case, would he now be פטור for any future fines of כפל or ד' וה'. But if the גנב first said "לא גנבתי" and then עדים contradicted him and said he did steal and then he admits that he was טובח או מכר, he would still be חייב the קנס if עדים came, because this admission wasn’t מחייב himself any payment. רש"י explains that this is not a sincere הודאה, because he knows a מודה בקנס is פטור and his true intentions in admitting is only to exonerate himself from the ד' וה'. A real הודאה is only when the admission makes him liable to something. Here it was the עדים that implicated him, not his הודאה. The Achronim explain the Lomdus according to רב who holds a מודה בקנס is פטור even if עדים come after, is that once he is מודה, his הודאה also acts as a פטור and therefore even if witnesses testify later that he is חייב, his הודאה was a פטור and he may not be charged anymore. Whereas שמואל who argues on רב, holds that granted a מודה בקנס is פטור, but his הודאה is not a פטור, it’s just that ב"ד doesn’t have the כח to be מחייב him a קנס based on his admission. But if עדים testify later that he was טבח או מכר, Beis Din can now be מחייב him based on the testimony of the עדים