Ten seconds on wikipedia would have shown you that the use of this symbol to denote dollars and pesos predates (if just barely) the founding of the United States.

http://en.wikipedia.org/wiki/Dollar_sign

So, barring time travel, I suggest that your theory about a U superimposed on an S must be wrong (at least as a causal story).

I've got to learn to stop caring about the rampant America-centrism on H.N.