Shaw Post Office and PhoneTech, which is on Rochdale Road, said in a post on Facebook that a man entered the premises at approximately 4pm on Thursday "demanding cash".
During the incident, the post office said the man also claimed to have acid on him and pulled out a bottle.
The post added: "We basically told him to get lost and activated the alarms.
"He made a quick exit, empty-handed."
There were no injuries reported from the incident and the post office have since confirmed Greater Manchester Police officers visited the premises to take evidence.
GMP have been contacted for any update.
