Robbers threatened a driver with a knife and stole his cash box in the car park of Sainsbury's, in Northolt Road, South Harrow.
The man, who works for a security company, was returning to his van when a group of men attacked him last Friday.
He was thrown to the floor and the three men ran off with the cash in the direction of Corbins Lane.
An investigation has been launched by the Flying Squad at Finchley.
Anyone with any information call 020 8358 1751 or Crimestoppers on 0800 555 111.