I know I've done the research and the coloring slash long fins severely throw me off, I saw them and fell in love them they seem like a very scared/ shy like type of fish.Hmm, I don't know! lol They seem to have the body shape and finage of nematocharax venustus (which is amazing that your LFS uses scientific names at all!) but the white at the base of the tail fin certainly doesn't match what I found on google. My guess is that it is a nematocharax species though. But wait for someone smarter to me to come along.
Edit: BTW is that black diamond blasting sand? Looks too dark to be flourite black sand.....not enough amber to be Tahitian moon sand and not perfect enough to be colorquartz.
I have seen them before, just do not remember where or what.
Has the tail color of the "Dawn Tetra" ,
Well the tail color looks spot on! But the dawn/pandas (same fish) don't seem to have the bump on their back like these guys and these guys look to have more slender longer dorsal fins, like the nematocharax venustus.I think I've seen these or very similar called "panda tetras". They may color up as they adjust to your tank & maybe grow a bit. Keep us posted