A charity box was taken from a gift shop in uphill Lincoln.
The incident happened on Bailgate on April 22 at around 1.45pm.
A man visited the Look No Further shop and demanded the the Help For Heroes collection box on display. It was given to him, and then he fled.
He is described as a white man with olive skin, around 5 foot 6 in height and of slim build and with short, very dark brown or black hair.
He was wearing a grey hooded jacket and dark blue jeans.
Police are appealing for anyone who may have witnessed this incident, and are also trying to find a man who was in the shop at the time.
He is described as an elderly man, around 80-years-old, and using a walking stick.
Anyone with information should contact PC Atkin at Lincolnshire Police, quoting incident 220 of April 22 2014 on 01522 885237.
Alternatively contact Crimestoppers anonymously on 0800 555 111.