To add to Aaron's answer:
The first retail store keeper in Monterey was probably Don David Spence, born 24 October 1798 in the parish of Huntly, Scotland to David Spence and his wife Helen Stewart:
(registration required to access this birth record. Due to copyright restrictions I am unable to post the image)
He died 18 February 1875 in ...
The first two images below are samples from a book which appears to be a record of payments in kind made to soldiers in the San Francisco Presidio in 1817, which perhaps confirms the suggestion in the question that they were unpaid during the war.
The document in the following images appears to show the payments made to the officers and men in the San ...