As a juvenile it's almost impossible to say with any certainty. It's also nearly impossible to tell while the fish is stressed laying on its side in a dish or held in your hand.
Where was the fish collected? What made you trust this person in the first place and why would you not trust what they sold you now? Just because of some lightness in the black?
All zebrasoma tangs are known to do this with their coloring going lighter when they're stressed or at night when the lights are out.
To me, that fish does not look like a scopas. It also isn't black as night in the pictures you provided so I get why you're ready to try getting your money back. Honestly, if the guy said it's a black tang and, at the time, you were fine with throwing down $800 for it I'd stick with your initial gut instincts. Let the fish settle in and report back to us how it looks in good lighting with a full belly.