[acqq]

Specifically, the US dollar exists only since 1785 and the sign $ was already used before for Peso according to Prof. Cajori who examined the West Indian manuscripts dated 1760 to 1778 http://books.google.at/books?id=4ykDAAAAMBAJ&printsec=frontc...

[wyck]

Here is a $ sign from pre 1700 Spanish coin http://www.ancientresource.com/lots/shipwreck-pirate-coins.h... ( #CS20730 ). There are more of these that pre date that, so this article is fairly opinionated.

[acqq]

And here's the sample of what Prof Cajori presented, a letter from 1778 (that is 7 years before the US dollar was introduced by The Grand Committee of the Continental Congress: http://www.usmint.gov/education/historianscorner/?action=tim...).

http://s23.postimg.org/p1tie5297/peso_as_dollar.png

Thanks to nemo: https://news.ycombinator.com/item?id=9019062