Colleagues of the victim called police after he shot him with a handgun at an eatery in the eastern Paris suburb of Noisy-le-Grand on Friday night, the source said.
Attempts to revive the 28-year-old failed and he died at the scene.
The gunman, who witnesses said lost his temper "as his sandwich wasn't prepared quickly enough" for his liking, fled the scene.
Police have opened a murder investigation.
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