There's actually nothing I saw that says you can't write on the money (again, as long as you don't make it unfit for circulation). This is mainly put in place to combat counterfitting. So, putting a "0" behind the "1" on a dollar bill would make it unfit for circulation for example.
I don't know if you've ever heard of something called http://wheresgeorge.com but this site, for a long time, has been tracking dollar bills by placing a stamp on them. If you find a bill with this stamp (the stamp leaves their website on the bill) you go to the site and type in its serial number to see where it has been. This site has been talked about on television so it's not some underground illegal dollar stamping ring hiding from the law. They've been around for years and obviously haven't been shut down by the government nor have they received any negative attention as far as I know.
"In April 2000, the site was investigated by the United States Secret Service, which informed Eskin that the selling of "Where's George?"
rubber stamps on the web site is considered "advertising" on United States currency, which is illegal under 18 U.S.C. § 475. The website's administrators immediately ceased selling the rubber stamps and no further action against the site was taken. At least one spokesperson for the US Secret Service has pointed out in print that marking US bills, even if not defacement, is still illegal for other reasons under 18 U.S.C. § 475; however, a Secret Service spokesman in Seattle, Washington, told The Seattle Times in 2004: "Quite frankly, we wouldn't spend too much looking into this."
--http://en.wikipedia.org/wiki/Where%27s_George%3F
So, I say, go for it actually.
The part about advertising on currency may apply, especially if "Fusion
Cash" is part of what's written on legal tender, (insofar as FC has no control over who writes a referral link on money, neither are they currently encouraging it).