I found Robert. He was a clerk in the Quartermasters Dept. of the War Dept. posted to Camargo, Mexico. He earned $60 a month.
The information was in a publication which may be a great resource for many people and yet wouldn't be found with a search on a person's name because the text isn't searchable. The name of the document is:
Register of all Officers and Agents, Civil, Military, and Naval, in the Service of the United States on the Thirtieth September, 1847, with the names, force, and condition of all ships and vessels belonging to the United States, and when and where built; together with the names and compensation of all printers in any way employed by Congress, or any department or officer of the government. Prepared at the Department of State, in pursuance of Resolutions of Congress of April 27, 1816, and July 14, 1832. Washington: J. & G.S. Gideon, Printers. 1847
It was apparently published every year in the 1830s and 40s and maybe even later. Hope this will help others in their search for ancestors.