This is one of those questions that probably have several answers. Mine comes from the George Washington exhibit I just went to last week. Another post on that.
At the exhibit were several exhibits on colonial times. One of them mentioned that there was coinage of many types and countries being used at the time, one of the most popular being the Spanish Dollar. Fur traders would pay hunters one dollar for each buck they brought in. Eventually 'buck' became slang for a Spanish Dollar.
What story have you heard?
Rick Potter