Thames Valley Police is releasing a CCTV image of a man officers would like to speak to about a robbery at a shop in Bletchley.
The offender told the victim they were going to rob the Best-one store in Melrose Avenue at around 7.35pm on Friday 22 October.
The victim sounded the audible alarm but the offender told them to put money in a bag and threatened to use a knife.
The victim put £800 in cash in a blue plastic carrier bag and gave it to the offender who then left the store.
Investigating officer Detective Constable Lisa McKinley, based at Milton Keynes police station, said: “We are releasing the image of this man as we believe they may have information that could assist the investigation.
“Anyone who recognises this man, or if you are pictured, please get in touch by calling 101 or making a report online, quoting reference 43210477784.
“Alternatively, if you wish to remain anonymous, you can contact the independent charity Crimestoppers on 0800 555 111 or via their website.”