1) The Brisker Rav lived in Eretz Yisroel. Why would he have $50 bills?
2) If he gave it, how may he take it back?
3) Why would he put in a $50 bill and not a $1 (or a piece of paper, or hair, for that matter)?–or was the point that the money could be kept, but only if he uses it?
4) How does the fact that the bill was still there prove that he never opened it?
5) A lot of people have a lot of s’forim which they use only from time to time. The fact that after a few months, the boy was still finishing shas and hadn’t yet the opportunity to peruse the Rav’s Seifer doesn’t mean anything. If the seifer had been opened, we would be hearing something about our dor where 13 year old boys are learning Brisker Ravs before they know anything else.