In the event that a divorced woman comes to court and demands that she be paid her ketuba, but she admits that part of what was owed to her was already paid, if her husband claims to have paid the full amount we will not allow her to collect unless she takes an oath indicating how much she is still owed.
The Gemara quotes Rami ban Hama as comparing this to a standard case of modeh be-miktzat – someone who is brought to court by a debtor who claims to have lent a sum of money – and the modeh be-miktzat admits to having borrowed, but argues that the amount that he owes is less than what is being claimed. In such a case, the Torah requires the modeh be-miktzat to pay what he admits he owes and to take an oath that he does not owe the rest of the money that is being claimed. Responding to this comparison, Rava points out that our case is different in a number of ways. For example, the modeh be-miktzat (and similarly, all people who are obligated by the Torah to take an oath in court) use the oath to free themselves of payment, while in our case the oath is being used to secure payment from the husband. Rather, the oath in our case is not biblically mandated, it is a rabbinic enactment whose purpose is to ensure that the woman is very careful that she is demanding only as much money as is really owed to her.
Tosafot raise the question that we should believe her claim based on a migo. The idea of a migo is that a person is believed when he makes a claim, if he could have made a claim that would have been more advantageous to him. The argument is that if he was going to lie, he would have chosen the better lie, therefore we can accept the lesser claim that he is making as true. The Rosh explains that a migo cannot apply here, because our suspicion is not so much that the woman is lying, as that she is not precise in the amount of money that she is claiming. The Ritva argues that this cannot be the case of migo for the same reason that we do not apply migo in the case of modeh be-miktzat. The Sages understood that people are reluctant to tell a big lie and deny all, but are more comfortable telling a small lie. It would be too difficult for this woman to claim that all the money is owed to her if she had been paid, but she might be able to claim that part was still owed to her.