Over at Online Visions' Challenge section, their current challenge reads as follows:

The performer borrows a bill and has a spectator sign it. The bill then changes to another denomination. The spectator's signature is still on the bill, exactly as they signed it, and can be given to the spectator to keep.

I can't help but wonder if they're familiar with Nicholas Night's "Real Artist's Dream" (from his "I Hate To Perform For Magicians" lecture notes), which answers this challenge exactly.

In this routine, Nicholas talks about what magicians call the "Artist's Dream", in which a painting comes to life. He mentions that the REAL artist's dream is that his signature alone can make him money, noting that signed works of art by the masters often bring in higher prices than unsigned works by the same master.

He then borrows a $1 bill, folding it down into a "work of art". It's folded so now it's just a portrait of George Washington, and kept that way with a paper clip. The money is then put in an "museum" (a wallet), so that we can "wait 20 years or so for the value to go up." When the wallet is opened, nothing is different. The portrait is still only worth $1.

Pointing out that this is because the work was never signed, the value didn't increase. The bill is signed by the spectator. With a wave of the hand, the $1 bill is changed to a $5 bill, with the signature still on the bill!