Resources for Kesubos 102
The גמרא brings a מחלוקת אמוראים between ר׳ יוחנן and ריש לקיש as to what is the הלכה when someone says חייב אני לך מנה. ר׳ יוחנן says he is חייב and ריש לקיש says he is פּטור.There is a מחלוקת ראשונים as to how ר׳ יוחנן understands that the חיוב works. רש"י in ד"ה לעולם understands that the person produces a שטר written by the other person that is essentially an IOU that says, literally, “I owe you $100” without saying a name or signing. Theמחלוקת is whether that שטר is enough to convince us that the writer who gave this over to his friend in front of witnesses wasn’t joking.
תוספות in ד"ה אליבא quotes ר"ת who disagrees and says that the person never actually owed the money but rather is trying to be מחייב himself the money now via this שטר which he wrote himself, has his name, but has no witnesses listed. In modern society we call that writing a check. The מחלוקת is whether you can be מחייב yourself with a שטר when no קנין was done. תוספות asks how can you be קונה the חיוב with a שטר when מטלטלין aren’t נקנית בשטר? He answers that since the person went to the trouble of writing the שטר we can assume he was גומר דעת. The תומים in סימן מ׳ אות א says that the reason תוספות needs to come on to this סברא is because the case is where there aren’t עדים on the שטר. However, if it had been a real שטר with עדים he would have no question since you can be גובה from נכסים משעובדים with a שטר so a קנין קרקע of sorts is involved in which case a שטר would work. This is why תוספות says the question as “how can he be קונה בשטר זה”. The ר"ן seems to have learned like תוספות but isn’t bothered by תוספות’s question of שטר not being able to be מקנה מטלטלין since this is just a התחייבות and not a קנין.
The third שיטה is that of theרמב"ם in הלכות מכירה פּרק י"א הל׳ ט"ו who says that there was no שטר at all. All that happened is that a man said in front of עדים “I owe you money and you guys are my witnesses” or “I owe you money and there is a contract”. Everyone knows there wasn’t a loan and yet it works to be מקנה the money. The קצות החושן in סימן מ׳ אות א brings the ר"ן who also asks on this that one can’t possibly be מחייב yourself just with words as we see from the case of a שומר חנם who wants to be מחייב himself to be a שומר שכר where the גמרא says he needs a קנין. So you see words cant possible make a קנין. The קצות answers for the רמב"ם that the רמב"ם was talking about “קנין אודיתא” which means that there is a קנין which is done by being מודה in front of עדים that you owe money. The source of the concept of קנין אודיתא is from a גמרא in בבא בתרא דף קמ"ט that says that איסור גיורא could be מקנה his possessions to his friend by being מודה that his friend owns it. However, the wording is key. If you say “I want to be מחייב myself something” that doesn’t work (which is the case of the שומר חנם trying to make himself a שומר שכר). It is specifically where you refer to the past and say “I already owe you money”. Also, the witnesses must be informed that they are to be witnesses (unless he mentions a contract). This is critical. Even if all agree what was said it isn’t a good קנין without witnesses being asked to be witnesses, like kiddushin which require עדים לקיום הדבר. Similarly, there is no גמירת דעת without that. What seems a little דוחק in this פּשט in theרמב"ם is that the רמב"ם says “הריני מחייב לך מנה” which doesn’t sound like the past.
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