Resources for Nedarim 90
The משנה says that originally a woman who told her husband the כהןthat she was נאנסה would get a גט and כתובה. It was then changed to say that she needs to bring a proof to her words since we are חושש that perhaps נתנה עיניה באחר. The ר"ן asks that if מעיקר הדין she should be believed, how does a חשש remove that איסור? His first answer is that we can say אפקיענהו רבנן לקידושי מיניה in which case she was never married when she was נאנסה. The מנחת שלמה in חלק א סימן ע"ו ד"ה גם points out that לכאורה this is a great way to get out of a kid being a ממזר if the lady is מזנה. She can simply tell her husband טמאה אני לך and if he doesn’t believe her then אפקיענהו רבנן לקידושי מיניהand the kid is no longer a ממזר! However, he says it will not work at all because even though מדאורייתא she is not married, מדרבנן she is מקודשת and the kid is a ממזר מדרבנן.
תוספות asks why don’t we say that the lady is still אסור on her husband because of שויה אנפשה חיתוכה דאיסורא? He brings from רבי אליעזר (ממיץ) that she never really made herself אסור because she is not מצווה to not marry a כהן—he can't marry her! So she never really made herself אסור. There is a famous חקירה as to how שויה אנפשה חיתוכה דאיסורא works. The מהרי"טsays it works מטעם נדר. The שב יעקב brought by the קצות in סימן פּ"ב אות בsays that it works מטעם נאמנות and that a person is believed about themselves כמאה עדים. In שב שמעתתא in שמעתתא ו׳ פּרק י"ט brings a proof from this רבי אליעזר (ממיץ) that it is מטעם נאמנות because if it was מטעם נדר then she can make a נדר to make herself אסור on others, but she isn’t believed in regards to others. So even if she didn’t make herself אסור on her husband, she made her husband אסור on her by saying she is טמאה לוif its מטעם נדר.
תוספות asks on רבי אליעזר (ממיץ) that the גמרא in יבמות דף פּ"͏ ע"ב says that if he is אסור to her then she is אסור to him. If so, by saying she was נאנסה she is making herself אסור to him as well. The חתם סופר here answers that the איסור the גמרא in יבמות is talking about is like a type of a לפני עורin the sense that if he isn’t אסור because he doesn’t believe her, then she isn’t אסור either.
The משנה says that according to the משנה אחרונה if the lady says נטולה אני מן היהודים he should be מפיר חלקו ומשמשתו. The טור in סימן רל"ד understands this to be a case where she makes herself אסור on everyone else. The בית יוסף asks she is משעובד to her husband so how can she do that? In that case the husband shouldn’t even have to be מפיר! רבי עקיבא איגר in תשובה סימן קס"ח says that her נדר is מפקיע מידי שיעבוד הבעל. Even though we normally don’t say that by a wife since אלמוה דשעבודיה דבעל, here where she is making herself אסור on the whole world and not just her husband we say it works.