Resources for Kesubos 88
1. The גמרא says that in a case where there is an עד אחד that a כתובה was paid, the lady will take a שבועה דרבנן and collect her כתובה. The גמרא then tries to figure out a way where the woman could be מחוייב a שבועה דאורייתא. The גמרא suggests that the husband pay the כתובה a second time in front of an עד thus having two עדים on one כתובה payment and making the first payment a loan which the עד אחד can now be משביע the wife a שבועה דאורייתא. The פּני יהושע asks an interesting question: why can’t the husband simply pay the כתובה to his wife, then turn around and have the עד say to his wife that she owes him the money he paid because he already paid her a כתובה a month ago and she would now need to swear a שבועה דאורייתא that she doesn’t owe the money! He answers with a חידוש: if an עד’s main purpose is to say someone is פּטור to pay then they can never be מחייב a שבועה דאורייתא since it isn’t similar to the case the תורה talked about.
2. What is the purpose of trying to get someone make a שבועה דאורייתא vs a שבועה דרבנן? The ראשונים give many answers, many of which seem to be based on the גמרא in שבועות דף מ"א that lists the differences between a שבועה דאורייתא vs a שבועה דרבנן. רש"י however says the difference is whether you need to say the name of Hashem and hold a ספר תורה when you make the שבועה. תוספות points out that רש"י seems to contradict himself since in גיטין he says that you say the name of Hashem when making a שבועה even though the גמרא there is talking about a שבועה דרבנן. The הגהות מימונית in הלכות שבועות פּרק י"א אות ה answers the סתירה by saying that not all שבועות דרבנן are the same. By most שבועות דרבנן there is a need to say the name of Hashem. However, our גמרא is talking about a שבועת היסת that is just להפיס דעתו של בעל הבית. That is a much lower level שבועה and no שם ה׳ is required for that.
3. The גמרא says in the name of רב יהודה the famous halacha that כל האומר לא לויתי כאומר לא פּרעתי דמי. There is a well known מחלוקת between the קצות החושן and מהרי בן לב in סימן ל"ד אות ד about how the concept of הודאת בעל דין works. The מהרי"ן לב says that the person themselves is not really believed by the תורה in a case of הודאת בעל דין; rather everyone has the right to be מחייב themselves money if they wish. Therefore, if you admit you owe money you are just being מחייב yourself now to pay. The קצות disagrees and says that the תורה gave a person actual נאמנות about their money. The קצות brings a proof from our גמרא. In our case, if you say that the תורה believes a person about their money, then when they say לא לויתי then they are essentially saying they never paid and we believe the person. Therefore, if we can prove they did borrow then they must pay. However, if the way הודאת בעל דין works is through התחייבות then in our case the person who said he never borrowed was never intending to be מחייב himself money so why would he have to pay? The דבר יעקב has an interesting answer. There is another חקירה brought by רבי עקיבא איגר inתושבות סימן קמ"ט. There he discusses whether the way לא לויתי כאומר לא פּרעתי works is because of הודאת בעל דין כמאה עדים דמי or because it’s an אנן סהדי. The דבר יעקב suggests that the קצות’s question is predicated on the fact that לא לויתי כאומר לא פּרעתי is because of הודאת בעל דין. However, if it is because of אנן סהדי then the קצות’s question doesn’t start, and the מהרי"ן לב may have learned like that צד in רע"א.