The truck belonging to AT Systems was making a delivery to E-Z Checks Cash at a strip mall when a person approached a messenger who was with the vehicle, police Lt. Carlos Fojo said.
What felt like a gun was pointed at the back of the messenger, who was told to walk over to a car and throw the money in the backseat, the FBI said.
A getaway car fled with two robbers, and the messenger fired three shots as the car drove away.
The FBI questioned two guards and were analyzing surveillance video, Fojo said.
An official at AT Systems Inc., which describes itself as a cash logistics company, refused to comment.
Hialeah Gardens is just northwest of Miami.