Right now--until we find an earlier document--the Buckinghamshire Posse Comitatus of 1798 is the earliest document mentioning Richard Price, grandfather to Robert Price.
The Buckinghamshire Posse Comitatus of 1798 is document containing "A Register of the Names and Occupations of all persons residing within the County of Buckingham (not engaged in any Military capacity) between the Ages of 15 and 60 years, And also of the Number of Draught Horses, Waggons, Carts of Burthen, Wind and Water Corn-Mills, within the same ... pursuant to a precept issued .. for the better ascertaining the Posse Comitatus."
Posse Comitatus means "force of the country;" however, it was not really used to conscript men into the military, as Roy Stockdill explains on his GENUKI webpage "Newbies' Guide to Genealogy & Family History," The following is a very informative explanation of the Posse Comitatus:
"Under the Defence of the Realm Act , lists known as "Posse Comitatus" lists and "Levee en Masse" lists were made in 1798 and 1803/04 respectively. These Defence Lists, despite their name, were not lists of those intended for military service. Their intention was to organise reserves of men not already serving in a military capacity for the defence of Britain against a French invasion. They would have been needed to evacuate the civilian population, remove wildstock and crops from the path of the invaders, gather arms and equipment and deal with food supplies to the forces and civilian population." (http://www.genuki.org.uk/gs/Newbie.html)
Richard is listed as a servant in Little Marlow on p. 236 of a copy of the Posse Comitatus, edited by Ian F. W. Beckett and published by the Buckinghamshire Record Society in 1985. The transcription was made from the original manuscript which resides in the Centre for Buckinghamshire Studies, Aylesbury.
This document shows us that Richard was residing in Little Marlow at least as early as 1798 and to my knowledge is the earliest document mentioning Richard Price.