Did they inform you of the size that they reach? You could talk to the manager and tell him/her that you were mislead to believing this fish is appropriate for your tank which it clearly is not.
Some stores won't take them back because of the size they reach. How long have you had this fish? you can also contact other fish stores and ask them if they would give you a trad in for it, or even if they won't trade it, they may accept it in the hopes of more business.
My mom got caught the same way, she was told "it will grow to the size of the tank" with a red sailfin for a 20Gal, he's now in my 75 @ 13.5" he was 12" when I moved him out of the 20. My LFS keeps asking me to bring it in for sale..