It happened on October 28 at the Family Dollar Store on the 2600 block of Germantown Avenue.
Surveillance video shows the suspect walking into the store shortly after 5 p.m.
He grabs a hammer from one of the shelves and walks behind the counter, demanding a 19-year-old female employee to open the cash register.
Click here to watch the surveillance video
When she told him she couldn't open the drawer, the suspect can be seen hitting her in the arm with the hammer and pushing her to the side.
He throws the cash register to the ground and a second employee walks behind the counter to confront him.
The clerk opens the register and the suspect grabs the cash before fleeing the scene.
The store employee suffered a minor bruise to her left arm.
Police describe the suspect as a black male, 28 to 35-years-old, 5'6" tall and 150 lbs. with a thin build and beard. He was wearing a black hooded sweatshirt and blue jeans.
Anyone with information is asked to call police at 215.686.TIPS (8477) or text a tip to PPD TIP (773847).