In the Rambam laws of marriage (chapter 16, law 25), the Mishna in Ketuboth at the beginning of chapter 2 (page 15b) is brought. [If a woman says she was married as a virgin, and the husband says as a widow, her ketuba is one hundred, and he takes an oath since he admits to part of the obligation.] Rav Shmuel Rozovvski of Yeshivat Ponovitch, Rav Shach and Rav Nachum of the Mir all deal with that law as it relates to Bava Metzia page 4b; admission in part causes the obligation of an oath. I thought that perhaps the law in that Mishna of Ketuboth is talking about a place where they write a ketubah [and thus if it is not shown, he could have said I paid it]. Also, one can say that the Rambam holds of an opinion (that I have never heard suggested before), i.e., that one can take an oath on a case where there is shiabud of land but one can not bring a sacrifice on that oath if he swore falsely. the fact is in laws of marriage in our law here, the Rambam does not mention the principle one does not swear on denial of shiabud (a lien or obligation) of land. But he does mention it in laws of oath concerning bringing a sacrifice. also, it is possible to note here that the ketuba is a document and all documents according to the Rambam are from the words of the sages. Therefore, any shiabud of land in it is derabanan.--------------------------In the רמב''ם הלכות of אישות (chapter 16, law 25), the משנה in כתובות at the beginning of chapter 2 (page 15b) is brought. [If a woman says she was married as a virgin and the husband says as a widow, her כתובה is one hundred זוז היינו מנה אחת ,and he takes an oath since he admits to part of the obligation.] זה relates to בבא מציעא דף ד' ע''ב; admission in part causes the obligation of an oath. I thought that perhaps the law in that משנה of כתובות is talking about a place where they write a כתובה [and thus if הכתובה is not brought forth, he could have said, “I paid it”]. Also, one can say that the רמב''ם holds of an opinion (that I have never heard suggested before), i.e., that one can take an oath on a case where there is שיעבוד of land, but one can not bring a sacrifice on that oath if he swore falsely. The fact is in laws of marriage in our law here, the רמב''ם does not mention the principle "One does not swear on denial of שיעבוד (a lien or obligation) of land." But he does mention it in laws of oath concerning bringing a sacrifice. Also, it is possible to note here that the כתובה is a document, and all documents according to the רמב''ם are from the מדברי סופרים. חיוב כתובה לחוד, שטר כתובה לחוד Therefore, any שיעבוד of land in it is דרבנן.