I think what you may need is something along the lines of
"Received with thanks the sum of £150.00 in full and final payment for 1951 Sunbeam S8 motorcycle registration number ABC 123. Signed A Shark, motorcycle dealer."
You may interpret this kind of document as a Bill of Sale in the USA, so if you think it may help, put that as a title for the document.
A suitable stamp of some kind would add authenticity, as would headed notepaper.
No doubt the dealer you purchased the bike from will be delighted to assist you.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.