The incident happened 1:00 p.m. Tuesday at a T-Mobile store at 1884 State Road in Upper Darby.
Police say a 59-year-old Philadelphia man had stopped in with a question about his bill.
"Apparently he was double-billed. During the course of the verbal exchange, it becomes heated," said Upper Darby Police Superintendent Michael Chitwood. "The next thing you know it becomes physical."
At one point the man rips off the name tag of the 21-year-old T-Mobile clerk, Darnell Schoolfield.
"As the victim is going out of the store after the altercation, you can see store clerk stab him in the side," Chitwood said.
The victim was able to drive himself to the police station to report the assault, not realizing he had been stabbed.
"He looks down and sees blood pouring out of his side and said 'Oh my God, I'm stabbed," said Chitwood.
The man who was stabbed was originally listed in critical condition but has since improved.
As for Schoolfield, police say he fled the scene and police are still looking for him. Once he is arrested, police say he will be charged with attempted homicide and aggravated assault.
T-Mobile put out a statement saying the company was "shocked" and "saddened" by what happened.