The difference between an oath required by the Torah and an oath required by the rabbis TB Ketubot 87

The Mishnah on daf TB Ketubot 87 enumerates five cases when the wife needs to take an oath to collect her ketubah. The fourth case is: “If one witness testifies that her marriage contract is paid, how so? If her marriage contract was a thousand dinars, and her husband said to her: You already received your marriage contract, and she says: I did not receive payment, and one witness testifies about the marriage contract that it is paid, she can collect it only by means of an oath.” (Sefaria.org translation)

Rami bar Ḥama and Rava disagree about the very nature of this oath. Rami bar Ḥama holds that this oath is required by the Torah while Rava holds that this oath is only required by the rabbis.

“Rami bar Ḥama thought to say that this is an oath required by Torah law, as it is written: “One witness shall not rise up against a man for any iniquity, or for any sin” (Deuteronomy 19:15). From here it is inferred: It is for any iniquity or for any sin that he may not rise up, i.e., the testimony of one witness is not enough for these purposes, but he may rise up for an oath. And the Master said: In any place, i.e., situation, where two witnesses are able to deem one liable to pay money, the testimony of one witness obligates him to take an oath.

“Rava said: There are two answers in the matter, in refutation of your argument: One response is that anyone who is obligated to take an oath that is enumerated in the Torah takes an oath and does not pay. By Torah law, one takes an oath only to exempt himself from payment, and in this case she takes an oath and takes her money. And furthermore, there is a principle that one does not take an oath with regard to a denial of a lien on land.

“Rather, Rava said: That oath was instituted by rabbinic law, in order to put the husband’s mind at ease. Since a witness contradicts her claim, the Sages imposed an oath upon her so that the husband would be sure that he is not giving away his money for no reason.” (Sefaria.org translation)

The Torah only requires a person to take an oath in three circumstances; 1, the court requires an oath be taken; 2, when the person admits to part of the claim (מוֹדְיָא בְּמִקְצָת.); 3, when a watchman or guardian (שומר) has to testify about the loss incurred under his watch. What’s the practical difference between an oath required by the Torah and an oath required by the rabbis?

There are two major differences. With an oath required by the rabbis, the oath can be flipped and the other side must swear to the facts. We saw an earlier case where the witness was known not to be truthful so the other side testified to the facts. One cannot flip the oath to the other side when the oath is required by the Torah. If for some reason, the person cannot take an oath he has to suffer the consequences. For example, if the watchman can’t to take an oath that the loss was not his fault, he has to make up for the loss out of his own pocket. The second major difference is a person holds a sefer Torah only when swearing an oath required by the Torah.