אביי said, the עדות by their signing acquire it for him - אביי אמר עדיו בחתומיו זכין לו
Summary:
רש"י explains that if there were not a set date for the changing of the king’s year, people will not know whether the date on the שטר precedes the loan and it will be a שטר פסול, or if the loan preceded the שטר and it will be a שטר כשר. Based on our גמרא here תוספות rejects this, for even if the שטר precedes the loan it is not פסול since עדיו בחתומיו זכין לו. According to תוספות the need to set a date is in case the סופר may mistake the kings ascent by one day (or so), and then it will indeed be a שטר מוקדם because the date on the שטר will precede the actual signing of the שטר.
Summary:
אביי independently maintains that עדיו בחתומיו זכין לו. He also independently maintains that in our משנה there can be no concern of שמא כתב ללות בניסן.
This is the reason; for we are concerned for payment and a swindle - היינו טעמא דחיישינן לפרעון ולקנוניא
Summary:
People will not cause undue loss to others unless they gain substantially. Therefore there is a concern of קנוניא for there is a substantial gain to the מלוה ולוה. There is no concern of making a new loan on the old שטר for there is significant loss to the לקוחות and minimal gain to the לוה. There is concern for כתב ללות ולא לוה for the לוה does not want to lose the פשיטי דספרי and there is no significant loss to the לקוחות for they are already aware of the loan משעת כתיבה.