As we have seen (daf 64b) in cases where a thief is obligated to pay kenas – the penalties of two, four or five times the value of the stolen object (over and above returning the object or its value to his victim) – if the thief steps forward and admits his guilt, then he will only need to return the object (or pay back its value); he will not have to pay the penalty – in the language of the Gemara, “Modeh b’knas patur.”
While this ruling is accepted by all, there is a difference of opinion whether this will be true even if other witnesses testify against him. Rav believes that even if witnesses are found who can testify about this situation, once the thief has admitted his guilt he is free from any obligations to pay the penalty. Others disagree, arguing that his admission is not sufficient to free him, since there were others who testified against him.
In our Gemara, Rav Hamnuna limits Rav’s ruling only to cases where the accused person’s admission obligated him in payment. If, however, his admission had no effect on what he would have to pay, it will not free him from the kenas should witnesses appear. Thus, if someone denied stealing an ox, and witnesses came to testify that he had stolen the animal, he is obligated to return it and pay the kenas of kefel (double). If at this point he admits that he also killed or sold the animal – which would ordinarily obligate him to pay an additional kenas – should witnesses come and corroborate that fact, he would still pay the additional kenas, since his admission had no effect on his obligation to pay.
Rashi explains that this is because we assume that the thief knew the law of modeh b’knas patur and was attempting to use it just to free himself from the obligation of the additional kenas. Others suggest that Rav’s ruling is based on the assumption that the thief’s admission is part of the process of teshuva – repentance – but in this case there may be ulterior motives, so we cannot rely on his admission.