We still don't know if they refunded your money -- did they?
In my opinion, both of you were wrong. You should have done the research into how to correctly ID these fish without question. Had that been me I would have offered the man $90.00. The shop in Cincy is selling JD's old red-tailed payara (probably 8" or so) for $90.00 I think, and I previously had bought a 6" Tatuaia for $65.00.
There's absolutely no way in hell River Wonders didn't know if it was an Armatus or Tatuaia. If you look at pictures, you can tell by head shape and body type, let alone fin color. Tats have more sloped and laid back heads, and slimmer and "weaker" looking bodies. Armatus are built "heavier" and thicker. Also, the Armatus wouldn't have been spooked if you'd shot a gun off beside the tank. I know, but you get my point.
They probably realized you weren't as knowledgeable as they were/are, and tried to pull a fast one on you. This won't be the last time this happens to a forum member, but there's a lot of us who won't forget it either.