ketuboth page 18 and 19. Rambam laws of loans, ch. 24

I think the Rambam explains a doc of faith or conscience that it is simply a lie, and therefore the witnesses that say'' this is our signature but it was fictious' are not believed. that is the Rambam understands that they are not believed because they are now testifying that they testified to a lie and therefore nothing they say can be believed. the doc itself is not validated, nor invalidated. The Gemara itself mentions the court might find another document with their signature and validate it by that. This approach suits the Rambam well. Reb Aaron Kotler brings Tosphot that holds a different reason they are not believed when they say the doc was fictious there was an oral understanding about it and that there was in fact no loan. Tosphot holds the reason they are not believed is that they admit the document was written according to law. Reb Aaron brings some explanations about Tosphot, but he decided the intension of Tosfot is based on Tosfot (pg. 18) that it is a migo in place of witnesses. That is the witnesses on the doc are considered separately from the two witnesses that now testify about that signature. Reb Aaron also shows that the ''I might have thought'' is a migo., not a case of the same witnesses that affirm also deny.----------------------------------------I think the רמב’’ם explains a שטר of אמנה or מודעא that it is simply a lie, and therefore the witnesses that say, "This is our signature, but it was fictious" are not believed. That is, the רמב’’ם understands that they are not believed because they are now testifying that they testified to a lie, and therefore nothing they say can be believed. The שטר itself is not validated, nor invalidated. The גמרא itself mentions the court might find another document with their signature, and validate it by that. This approach suits the רמב’’ם well. ר' אהרון קוטלר brings תוספות that holds a different reason they are not believed when they say the שטר was fictious )there was an oral understanding about it, and that there was in fact no loan(. תוספות holds the reason they are not believed is that they admit the שטר was written according to כהלכתא. NOW ,רב אהרן brings some explanations about תוספות, but he decided the intension of תוספות is based on תוספות (דף י''ח) that it is a מיגו in place of witnesses. That is the witnesses on the שטר are considered separately from the two witnesses that now testify about that signature. ר' אהרן also shows that the הווא אמינא is a מיגו, not a case of the הפה שהיתר הוא הפה שאוסר.