Deputies say a call came in from an apartment complex on Spiral Road that a teenager's cell phone had been stolen.
Agents say once they investigated the case, they realized marijuana was involved.
TCSO says 19-year-old Frank Coston was going to buy the drugs from the minor when he decided he didn't want to pay for them - then pulled out a gun.
Coston faces numerous charges including armed robbery.
Both he and the teen are charged with marijuana possession with intent to distribute.