There is a law in the Gemara Ketuboth on page 19B and brought in the Rambam from Rav Nachman who said is that witnesses that say about a document with their signature that it was a document of “faith” or of “knowledge” (i.e., fictious) that they are not believed, and the doc is considered valid in spite of their later statement. However, we know from the Mishna that if they say they were children or that they signed by force, they are believed. But in a different law, we find that one can write a document for a borrower even though the lender is not with him if that document has a acquisition by handkerchief written in it. I already wrote once in my ideas in Bava Metzia that it was unclear to me if that document says there is a loan, (present tense), or there will be a loan, (future tense). Rav Shach points out in the Avi Ezri laws of loans 24 law one that the above two laws contradict each other and not cannot both coexist. [in one case you say they can write a fictious doc. In the other case you say they are not believed when they say they signed on one.] To me the fact that the Rambam brings both laws shows clearly that the Rambam holds the law that one can write a document for a borrower even though the lender is not with him means the doc says there will be a loan future tense, or that it means that in fact there already was a loan in fact (past tense) and that in reality there was a loan.-------------------There is a law in גמרא כתובותon page י''ט ע''ב from רב נחמן who said is that witnesses that say about a שטר with their signature that it was a שטר of אמנה or of מודעא (i.e., fictious) that they are not believed, and the שטר is considered valid in spite of their later statement. However, we know from the משנה that if they say they were children or that they signed by force, they are believed. But, in a different law, we find that one can write a שטר for a borrower even though the lender is not with him if that שטר has an acquisition by handkerchief written in it. I already wrote once in my ideas in בבא מציעא that it was unclear to me if that שטר says there is a loan, (present tense), or there will be a loan, (future tense). רב שך points out in the אבי עזרי laws of loans כ''ד law א' that the above two laws contradict each other, and not cannot both coexist. [In one case, you say they can write a fictious שטר. In the other case, you say they are not believed when they say they signed on one.] To me the fact that the רמב’’ם brings both laws shows clearly that the רמב’’ם holds the law that one can write a שטר for a borrower even though the lender is not with him means the שטר says there will be a loan future tense, or that it means that in fact there already was a loan in fact (past tense) and that in reality there was a loan.