I see the problem that Rav Shach and the other great roshei yeshivot were dealing with in Rambam laws of marriage 16:25. If there is doubt about how much the ketubah was for [200 or 100], and he claims it was for 100, he takes an oath since he admits part of the amount that is owed, and pays 100.the problem comes from two directions. The Rambam writes in (chapter 16 law 21) that in a place where the custom is to write a ketuba and the woman does not bring one forth, she receives nothing. On the other hand, if a ketuba is not written then it is an act of the court and he can not claim he paid it. and there is no oath for admission in part when the part that he admits he could not have denied. I think Rav Joseph Karo (Shulchan Aruch) intends to answer this question when he brings this law in Even HaEzer 96, law 3, he says the ketubah was lost by in a way that she could not have prevented it, for example the ship it was on sank at sea, or it was stolen by armed robbers. I only had a small amount of time to look at Reb Aaron Kotler on this subject, and I think he intends to answer this question thus. The husband can claim he gave her objects of value to keep to pay for the ketuba while they were still married. He can not claim he paid it after the divorce, but this type of claim might be believed. Therefore, if he does not claim it, but admits in part, then it counts as admission in part, and he takes an oath.---------------------------I see the problem that רב שך and the other great ראשי ישיבות were dealing with in רמ''ם הלכות אישות ט''ז הלכה כ''ה. If there is doubt about how much the ketubah was for [200 or 100], and he claims it was for 100, he takes an oath since he admits part of the amount that is owed, and pays 100. The problem comes from two directions. The רמב''ם writes in (פרק ט''ז הלכה כ''א) that in a place where the custom is to write a כתובה and the woman does not bring one forth, she receives nothing. On the other hand, if a כתובה is not written then it is an act of the court מעשה בית דין and he can not claim he paid it. And there is no oath for admission in part when the part that he admits he could not have denied. I think רב יוסף קרו (שלחן ערוך) intends to answer this question when he brings this law in אבן העזר צ''ו הלכה ג', he says the ketubah was lost by in a way that she could not have prevented it, for example the ship it was on sank at sea, or it was stolen by armed robbers. I only had a small amount of time to look at רב אהרן קוטלר on this subject, and I think he intends to answer this question thus. The husband can claim he gave her objects of value to keep to pay for the כתובה while they were still married. He can not claim he paid it after the divorce, but this other type of claim might be believed. Therefore, if he does not claim it, but admits in part, then it counts as admission in part, and he takes an oath.