I had thought of a possible answer to the question of the Raavad on the Rambam in laws of marriage chapter 16 law 25. The Rambam brings the mishna that if there is disagreement if a woman was a virgin when she got married, the ketubah is a hundred zuz, and the husband takes an oath [saying that she was not a virgin and therefore her ketuba is only 100 zuz, not 200.] because he is admitting to part of the obligation. The Raavad says there is no oath from the Torah here because it one does not swear on denial of a lien [shibud] on land. The answer to this question [I had thought] is that the Rambam is going like the opinion of the Ri Migash brought in the Shita Mekubetzet Bava Metzia page 4 who says that the principle that the Raavad brings is only when the document is present. Clearly it is not present here because if it was, we would know if it was for a virgin or not. However, this is impossible because in laws of loans 14:4 the Rambam writes if a lender brings forth a document of a debt and the borrower says he paid half and witnesses say he paid it in full, he takes an oath because he admitted to part of the debt and pays half. so here the document is before the court and in that case the Ri Migash says he ought to not be obligated to take an oath because it is a case of denial of an obligation of payment by means of land. However, on second thought, perhaps the Rambam holds a lien on land applies specifically to a ketuba which ought to be collected only with land. Thus, a loan might not be thought to be in the category of shiabud on land. So, for a loan, there would be an oath in all cases of admission in part. But for a ketubah, the only time there would be an oath is when there is no ketubah this is presented to the court. However, this answer needs a bit of modification since the Gemara in chapter oath of a pledge and the Rambam in laws of oaths chapter 8 law 13 hold that a debt that is backed by a document is also considered a lien on land. so, the only exception to this would be a loan that is not backed by a document.--- ------------------I would also like to bring here an answer to the question of the ראב''ד on the רמב''ם in laws of אישות chapter 16 law 25. The רמב''ם brings the משנה that if there is disagreement if a woman was a virgin when she got married, the ketubah is a hundred זוז, and the husband takes an oath because he isמודה במקצת . The ראב''ד says there is no oath from the תורה here because it one does not swear on denial of a lien [shibud] on land. The answer to this question [I had thought] is that the רמב''ם is going like the opinion of the ר''י מיגש brought in the שיטה מקובצת בבא מציעא דף ד' who says that the principle that the ראב''ד brings is only when the document is present. Clearly it is not present here because if it was, we would know if it was for a virgin or not. However, this is impossible because in הלכות מלווה ולווה 14:4 the רמב''ם writes if a lender brings forth a document of a debt, and the borrower says he paid half, and witnesses say he paid it in full, he takes an oath because he admitted to part of the debt and pays half. So here the document is before the court, and in that case the ר''י מיגש says he ought to not be obligated to take an oath because it is a case of כפירת שיעבוד קרקעות.----However, on second thought, perhaps the רמב''ם holds a שיעבוד on land applies specifically to a כתובה which ought to be collected only with land. Thus, a loan might not be thought to be in the category of שיעבוד on land. So, for a loan, there would be an oath in all cases of מודה במקצת. But for a כתובה, the only time there would be an oath is when there is no כתובה which is presented to the court.-------However, this answer needs a bit of modification since the גמרא in chapter "שבועת הפיקדון" and the רמב''ם in הלכות שבועות פרק ח' הלכה י''ג hold that a debt that is backed by a document is also considered a שיעבוד on land. So, the only exception to this would be a loan that is not backed by a document. כגון הדיון ברמב''ם הלכות מלווה ולווה פרק י''ג הלכה ד' איפה שאין שטר