The man was set upon by a gang in a car park behind the AIB bank off Main Street.
The man was struck in the head and knocked unconscious before the gang made off with a “substantial” amount of cash.
The attack happened just after 1.30pm on what is a busy car park just off the town’s main shopping area.
Their victim was found in a pool of blood by a passer-by a short time later between his own van and a parked car.
Gardai suspect the gang had the car-park staked out and were waiting on the well-known local businessman to make a lodgement.
Inspector Pat O’Donnell confirmed that Gardai are viewing CCTV footage of the crime which happened on Friday last.
Gardai said they did not want to release details of the attack because the man was still bring treated in hospital.
Inspector O’Donnell said the crime was “nasty” and appealed to anybody who may have been in the area at the time to contact them at Letterkenny Garda station.
He also made an appeal to business-owners lodging cash to be vigilant at all times.
“This incident has shown the lengths some people will do to get their hands on cash.
“This was a vicious and nasty attack which left this completely innocent man in hospital.
“We would ask people who are lodging cash to be vigilant and to take whatever precautions are necessary to ensure their safety,” he said.