On Bava Metzia 3a, it talks about a concept that if a plaintiff claims Bob owes him 100 and Bob says no I don’t, nothing is done, but if Bob admits and says “I only only you 50, not 100,” (or if witnesses say he owes at least 50) then Bob has to swear that he doesn’t owe the other 50.
In mishnayot Ketubot, there’s this concept of הפה שאסר הוא הפה שהתיר. For example, if a woman says “I was taken captive but am pure,” we believe her because she gave herself the thought of being impure by telling us she was a captive, so therefore the latter half of her statement is believable (otherwise if she was impure, she wouldn’t have mentioned being captive in the first place to take away suspicion). But if witnesses say “she was taken captive” and she says “no I’m pure,” we now don’t believe her because she didn’t אסר herself.
Anyway, while these two things don’t exactly line up, my question is why in the case of when Bob admits he owes 50 do we not believe him about the other 50 and we make him swear? Him admitting to owing 50 seems to be a case of הפה שאסר and you would think that we believe he doesn’t actually owe the other 50, because otherwise he would’ve just said he owes nothing and then he’d be fine and have no suspicion against him. So why doesn’t the concept of הפה שאסר apply to the Bava Metzia case?