A mishnah on today’s daf states:
One who vitiates her marriage contract can collect only by means of an oath.
This is a little confusing, so a few lines later the mishnah offers an example:
How so? If her marriage contract was 1,000 dinars, and her husband said to her: “You already received your marriage contract,” and she says: “I received only one hundred dinars,” she can collect her marriage contract only by means of an oath.
A woman who claims she has only received partial payment on her ketubah is not entirely trusted without an oath to back up that claim. The reason is not clear in the mishnah, but the debate in the Gemara, about whether this oath is required by Torah law or rabbinic law, sheds some light. First, the claim that it is an oath required by Torah law:
Rami bar Hama thought it is an oath required by Torah law, as the husband claims that he paid her 200 and she concedes to him with regard to 100. This is a partial admission of the claim, and the principle is that whoever admits to part of a claim must take an oath (in order to receive the remaining amount, according to Torah law).
The numbers here are different from the mishnah, but the principle is the same: She claims she’s only received part of what she is owed in her ketubah. Rami bar Hama holds that the oath she must now take to that fact is required by Torah law, which stipulates oaths in cases of partial payment. His position is quickly challenged, however, by Rava, who points out:
Anyone who is obligated to take an oath that is enumerated in the Torah takes an oath and does not pay.
In other words, says Rava, Torah-mandated oaths are taken only to exempt someone from payment. One takes such an oath that one does not owe money, but one does not take such an oath to collect money. Rather, Rava claims, the oath she must take is prescribed by the rabbis, not the Torah, and he explains why:
One who pays is precise; one who was paid is not precise.
This is a psychological insight: One who has paid money will be far more likely to remember the exact sum that was paid out than one who collects money. It’s easier to forget receiving a payment than being forced to cough up a large sum. Therefore, in this particular scenario of partial payment, her memory is suspect and we require her to take an oath to be sure she has the right number. Presumably, the seriousness of the oath will inspire her to think hard and be sure of what she received.
Oaths (shevuot) and vows (nedarim), as we have already seen, were taken seriously by the rabbis. In fact, the weight of these is so ingrained in Judaism that the rabbis discouraged taking them unnecessarily, and sometimes at all. Some believe that this is the origin of the Kol Nidrei prayer, which dates to the period of the Geonim (right after the closing of the Talmud) and is recited in synagogues on Erev Yom Kippur. Kol Nidrei (literally: “All the Vows”) formally absolves every member of the congregation of any vows they make in the coming year (with some exceptions, depending on who you ask).
Another artifact of the seriousness with which Jews approach oaths and vows is the common verbal expression among Jews of tacking on the words bli neder — without a vow — to verbal promises. This expression makes sure the listener doesn’t mistake the promise for a solemn vow. The intent is: I hope and fully anticipate doing whatever has been promised, but because it is considered a grave error to fail to fulfill a vow, I’m not swearing to anything.
Read all of Ketubot 87 on Sefaria.