He did not say, like an unattributed individual - כסתם יחידאה לא אמר
Summary:
The meaning of כסתם יחידאה לא אמר is either that if there are two contradictory משניות we do not follow the יחידאה, or this statement was only in the הוה אמינא.
Summary:
We can write a הרשאה even on מטלטלי דכפריה since the ruling is like the א"ד and we are not concerned for מיחזי כשיקרא. We may even write a הרשאה on a loan since he is a שליח, for whom the חכמים instituted as if he acquired it בקנין גמור.
