You have a borrowed banknote. Ask a spectator to sign it. You put it in an enveloppe and you burn it.
Take a brass tube from your pocket ang give it to the worried spectator. The is locked with a padlock so show him the keys on the table.
He open the tube and...he find the banknote inside !Each parts of the equipment can be examined.
Faites signer le billet par son propriétaire : il sera d'autant plus content de le retrouver.