Talmud for Ketubot 9:7
הַפּוֹגֶמֶת כְּתֻבָּתָהּ, לֹא תִפָּרַע אֶלָּא בִשְׁבוּעָה. עֵד אֶחָד מְעִידָהּ שֶׁהִיא פְרוּעָה, לֹא תִפָּרַע אֶלָּא בִשְׁבוּעָה. מִנִּכְסֵי יְתוֹמִים וּמִנְּכָסִים מְשֻׁעְבָּדִין וְשֶׁלֹּא בְפָנָיו, לֹא תִפָּרַע אֶלָּא בִשְׁבוּעָה:
If a woman "impairs" her kethubah (see 9:8), she exacts payment only with an oath. [When one is paid by another, he is not (always) careful to ascertain that he has been paid the exact amount; and this one (the woman), since she was (by her own admission) paid in part, might have been paid in full. And the rabbis imposed an oath upon her so that she be exact (in her reckoning).] If one witness testifies that it has been paid, she can exact payment only with an oath. [This is a rabbinical ordinance designed to set the husband's mind at rest. And these oaths, though instituted by the rabbis, are like Torah oaths, requiring the holding of a (sacred) object. For all oaths instituted in the Mishnah are like Torah oaths.] (If she came to claim her kethubah) from the property of the orphans, or from bound property, or not in his (her husband's) presence, she can exact payment only with an oath. [For if one would claim from the debtor himself, and he would say: Swear to me that I did not pay you, he would have to swear. And we argue for the receiver (of the property), viz.: Perhaps if she had claimed from the debtor (her husband), he would have said: "Swear to me that I did not pay you," and she would have had to swear. Here, too, she has to swear.]