The 58-year-old was walking towards Allied Irish Bank on West George Street, Glasgow, at about 1.30pm yesterday when he was attacked from behind.
The businessman had just walked past a black BMW car that was parked on the street when a man who was leaning into the window of the car turned and assaulted him and tried to steal a backpack which contained a "substantial" amount of cash, Strathclyde Police said.
The victim's friend, a 43-year-old man, who was sitting in his car waiting for him, saw the assault and got out of the car and ran across the street.
He pulled the attacker off his friend and they began fighting in the middle of the road.
The would-be thief then ran down West George Street where he got into the black BMW which had already left the scene and the car continued to make off towards George Square.
During the fight, the 58-year-old managed to make his way into the bank, where staff contacted police.
Both men suffered only minor injuries in the incident but were left very shaken.
The attacker is described as white, Scottish, about 50, 5ft 11ins, with cropped grey hair and a stocky build. He was wearing a brown jacket and dark trousers.
Detective Sergeant Scott Douglas said: "Thankfully neither of the men were injured in this assault and I am appealing for the help of the public to trace the men responsible.
"From inquiries so far we know that there was a white van stopped in front of the two men who were fighting in the roadway and saw what took place, and I am appealing for the driver to come forward as he no doubt will have information which can help us.
"I would also like anyone who was in West George Street around 1.30pm on Monday and saw the altercation take place or saw a black BMW parked on the street to contact me."