Stratis by Michal Kociolek and Roman Slomka

Their signed bill appears between the layers of a playing card.

You read that right. A bill is borrowed from someone in the audience and signed to ensure it can't be switched for a duplicate. After making the bill vanish, you then reveal that it has re-appeared in a truly impossible location.

The top of a playing card is ripped open to show that, in-between the individual layers of paper that make up the front and back of their card, is their bill. They can immediately take it back, leaving them with a one-of-a-kind souvenir that ensures they'll never forget this unbelievable experience.

You don't need to switch any bills or use duplicate signatures. The bill that ends up between the layers of their card, is genuinely their bill.