Series 1950 $5 Counterfeit?

I got a counterfiet 20 from a bank once.
No one had to tell me it was fake, but I was away
from the bank already when I figured it out. So I
had no proof. I brought it back to them and complained,
they did 2 things that seemed to contradict themselves...
The insisted there was no way they gave it to me, and
they said they had to keep it, but would give me 20
dollars for it. I don't know that they would have given
me the 20 if I hadn't claimed it was from them, or if
they were just trying to nip my story in the bud. ?