Timothy Smith, 57, was pushed during the robbery in The Drive, Hullbridge, at about 23:00 GMT on 19 March.
He fell heavily on to a low level wall and suffered a serious brain injury. He died two days later.
A 50-year-old man from Southend has been detained on suspicion of murder, robbery and possessing a firearm.
Detectives are appealing for anyone who saw a Ford transit van in or near Ferry Road, or two men at or near a bus stop on Ferry Road in Hullbridge on 19 March between 19:00 and 23:00 GMT, to contact them.
A man has been arrested on suspicion of committing a murder during an armed robbery in Essex.