What would you have done?

if the teller was nice to me, i would've returned the $20 even if the manager told me to keep it. i would hate to see someone getting into trouble over an honest mistake after they were nice and professional.

if the teller was an ass or unprofessional, i would've accepted the $20 and tried to get the teller in more trouble 'cause that's how i operate. i treat others the same way they treat me.

i think the way you handled it was pretty good. without knowing anything else that went on, i would have done exactly the same thing.