It happened Thursday night around 1:30 a.m. inside the In & Out Deli on the 5100 block of Germantown Avenue.
Surveillance video shows the suspect arguing with the clerk for several minutes before punching the Plexiglass counter.
Click here to watch the surveillance video
He finally breaks it and then grabs a bottle of liquor before fleeing the scene.
Police say he was last seen running north on Germantown Avenue.
The employee suffered an injury to his finger during the robbery.
The suspect is described as a black male with a beard and stocky build. He was wearing a red shirt, faded dark colored jeans and white sneakers.
Anyone with information on this suspect should call police at 215.686.TIPS (8477) or text a tip to PPD TIP (773847).