I think your words in red are correct. Here it is updated, with a few of my own added.
Know all men by these presents that I James Ard
of the County of Wayne and State of Kentucky and late a
private in Capt. Adam Vickery’s Company of the 15th
Regiment Kentucky Detached Militia in the service of the
United States under the Command of Major General John
Thomas in the year 1814 have for a valuable consideration
to and in hand paid by Drury Shrewsbury sold unto the sd
Shrewsbury the wages ____________from the United States
for six months serviced in sd Detachment I do hearby (sic)
authorize and empower the sd Shrewsbury foresaid And in my
name to ask demand and receive and from the paymaster
of sd Regiment the whole of the amount due me as aforesaid
and to be executed in my name to the said paymaster all
iou pay receipts and aquitances therefor Ratifying and
confirming every act and deed of sd Attorney in the presence
as fully and as effectually as if I were present and acting
for myself and to do and ____act the same
In witness where of I have here unto set my hand and seal
this 18th day of September in the year 1815
Signed, sealed, and acknow-
ledged in presence of
(J) aco. Duffey
Alice? Abie? Shrewsbury
I have uploaded the second page to James' profile, which I believe to be correct except for the names. Note the mark next to James' name, and almost the same mark next to 'Duffey'. Perhaps the attorney signed for both, since they were unable to write? Is it possible the mark that looks like a captial J by Duffey's name is Duffey's mark?
There is a private in James' regiment named John Duffey, I suspect he witnessed this for James. The other person is a Shrewsbury, possibly the attorney's wife or daughter?
After reading through this it appears as though this document does two things. It acknowledges that Attorney Shrewsbury paid James directly, probably a lesser amount than he was owed, in exchange for getting his pay immediately, and acts as a receipt. Then, it authorizes Atty. Shrewsbury to go collect the full amount due James from the Government, the difference being his fee.