On what are you relying, on this שטר; this שטר is merely a potsherd - אמאי קא סמכת אהאי שטרא האי שטרא חספא בעלמא הוא
Summary:
תוספות offers three interpretations why a מיגו is ineffective here:
a) It is a מיגו להוציא. b) It is a מיגו that is prefaced by a lie. c) He is a חוזר וטוען.
And the law is according to רבה concerning property - והלכתא כוותיה דרבה בארעא
Summary:
The רשב"ם maintains that ראב"א ruled like רבה בארעא [and like רב יוסף בזוזא]; since he was in doubt whom to follow. The reason in our case we award it to the מחזיק and not to the מרה קמא as in תו"ת; is because here it is a ספיקא דדינא.
However תוספות himself maintains that ראב"א ruled like רבה, but by זוזא it is a מיגו להוציא.