Reference for Ketubot 174:4

אלא אמר רבא מדרבנן דפרע דייק דמיפרע לא דייק ורמו רבנן שבועה עלה כי היכי דתידוק

Rather Rava said, [that the oath is only] of rabbinic status. For the one who pays is careful to remember the details while the one who receives payment is not, the Rabbis have imposed an oath upon her so that she would be careful to remember the details.

Jastrow