Three Collar Buttons

Published as 3 Collar Buttons in Best Stories, October 1927. Reprinted in the collection Aurelius Smith - Detective.
That a con job was underway was obvious but the problem facing Aurelius Smith was who was doing the conning in this matter of a newcomer to a small town out to buy real estate and using a check on a non-local bank to fund it. What did he put into the envelope as security?