The customer was robbed by two men on Saturday at around 10am in the Rosetta area of the Ormeau Road.
It is understood that the victim was on his way into the post office to deposit cash when he was attacked.
The police cordoned off the scene while officers, along with the fire and ambulance services, attended to the incident.
A police spokesperson said the man has been taken to hospital for treatment, but at this stage, it is believed the substance is nothing untoward.
The robbers made off in the direction of St John's Avenue with a sum of cash.
One of the men is described as being in his 20s and was wearing a red and white top.
Anyone who has any information about the theft is asked to contact the PSNI.