Use THEIR money to do magic!
A classic of magic with a borrowed and signed dollar bill!
Let your spectator take out his own dollar bill and sign it. The bill goes under the special black cloth which the spectator holds. The spectator also holds onto a padlocked brass tube. When the cloth is whisked away- the bill has vanished! And the signed bill is found- you guessed it!- inside the LOCKED tube that the spectator has been holding all along!