Shopkeepers in the Daventry area have been urged to be on their guard if they are asked to change large bundles of bank notes by customers.
During September, two Asian men entered a petrol station in the area and asked for a large bundle of £20 notes to be changed for £10 notes.
They then confused the cashier, who later noticed £40 was missing from the till.
It is thought the men may also have tried the tactic at other businesses in the area.
Anyone who knows anything about these incidents, can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.