The incident happened at business premises close to Maharaja Fabrics in Cemetery Road at about 7.30pm last Sunday.
He was leaving the premises when he was approached by three suspects who chased him back inside.
He was then assaulted and threatened with a gun before the suspects fled with the bag, which is believed to have contained a large amount of cash.
He suffered a minor head injury. Police believe it was a pre-planned attack and are appealing for witnesses.
Detective Inspector Mark Taylor, of Bradford District CID, said: “This would have been a frightening experience for the victim who was forced to hand over the bag at gunpoint.
“We believe this was a pre-planned attack and are keen to speak to anyone who saw any suspicious activity in the Cemetery Road area, possibly towards the junction with Thornton Road, at around the time stated.”
Call 101, quoting crime reference 13180616796, or use the live chat facility on the West Yorkshire Police website.
Or call Crimestoppers on 0800 555111.