Place a small wooden box down on the table. Borrow a dollar bill from a spectator and have him sign it with a magic marker. Fold it up as you ask, "Is this the dollar bill you gave me?" When he confirms, say "Thanks!" and put it in your pocket.
Continue, "You can have your dollar back, or you can trade it for what's inside the box." Point to the wooden box resting on the table. Whatever he chooses he's in for a shock... Slowly reach over with your empty hand and bring the box over to the spectator. "Please remove the lid." He does so and sees a folded bill inside. Dump it out and let him unfold it - it's his signed bill!
The Money Mystery Box seems impossible because no one went anywhere near the box. And it's easy to do with no sleight-of-hand necessary.
Any USA paper currency will work. $1 bills, $20 bills, even $100 bills - you name it!
The illusion is now 3D - your audience sees a creased, folded bill inside the box. Also, the box can be FULLY EXAMINED because the gimmick is recessed and concealed from view.