The Rambam writes in laws of testimony 3:4 that one cannot decide a case involving money unless you have two witnesses, not just their signature on a document. I think the Rambam might mean this to apply to all cases involving money. You might argue against this from the Gemara in Sanhedrin that asks how can late documents be valid? Do not we need the ability to cross-examine? How can we so unless there is an exact date on the document? The Gemara answers there is a decree so that the door will not be shut in front of those who wish to borrow. The questioner believes documents are valid by the law of the Torah. However, I think the Rambam understood that the answerer of the Gemara rejects that assumption, and holds instead that all monetary documents are derabanan, and that they are considered valid for the benefit of those who want to borrow. However, Rav Chaim of Brisk wrote that the Rambam hold documents that cause a acquisition are valid from the law of the Torah, but not documents that are merely proof that an acquisition has been made. Rav Shach brings a few proofs against that approach, and writes that the Rambam holds even monetary documents are also valid from the law of the Torah, but not documents upon which there is disagreement. When there is disagreement among the parties involved you need the actual witnesses.----------------------------------------------The רמב’’ם writes in laws of testimony פרק ג הלכה ד' that one cannot decide a case involving money unless you have two witnesses, not just their signature on a document. I think the רמב’’ם might mean this to apply to all cases involving money. You might argue against this from the גמרא in סנהדרין פרק א' that asks how can late documents be valid? Do not we need the ability to cross-examine דרישה וחקירה ? How can we so unless there is an exact date on the document? The גמרא answers there is a decree so that the door will not be shut in front of those who wish to borrow. The questioner believes documents are valid by the law of the תורה. However, I think the רמב’’ם understood that the answerer of the גמרא rejects that assumption, and holds instead that all monetary documents are דרבנן, and that they are considered valid for the benefit of those who want to borrow. However, רב חיים מבריסק wrote that the רמב’’ם hold documents that cause a acquisition are valid from the law of the Torah, but not documents that are merely proof that an acquisition has been made. רב שך brings a few proofs against that approach, and writes that the רמב’’ם holds even monetary documents are also valid from the law of the Torah, but not documents upon which there is disagreement. When there is disagreement among the parties involved you need the actual witnesses.-----One oof the proofs of Rav Shach against Rav Chaim is that a document of a loan just by itself does cause a acquisition, just like a document of a sale of a field, [and thus the claim of Reb Chaim disappears that it is just a document of proof that a loan happened]. That is true to many Rishonim including Rashi and the Ramban. However, not according to Rabbainu Chananel who writes in Bava Metzia page 13 side a about the document of acquisition that Rav Asi brings there that the acquisition was caused by a Kinyan Sudar, not by the document. Also, the Rashba holds even after the document was signed and is valid, still there is no obligation to make or receive the loan until money has been exchanged and therefore there to their opinion, a document of a loan can be called a document of proof, not a document that causes a acquisition.-----------One of the proofs of רב שך against חייםis that a document of a loan just by itself does cause a acquisition, just like a document of a sale of a field, [and thus the claim of רב חיים disappears that it is just a document of proof that a loan happened]. That is true to many ראשונים including רש''י and the רמב''ן. However, not according to רבינו חננאל who writes in בבא מציעא י''ג ע''א about the document of acquisition that רב אסי brings there that the acquisition was caused by a קניין סודר, not by the document. Also, the רשב''א holds even after the document was signed and is valid, still there is no obligation to make or receive the loan until money has been exchanged, and therefore there to their opinion, a document of a loan can be called a document of proof, not a document that causes a acquisition.