Next bit of advice is find yourself a new LFS! Quality ones will almost always take back fish within a reasonable time frame, particularly if it's a compatiblity issue with your existing fish--I know the one I work for does. They might not refund your money, but all the good stores I know will take in fish they sold, as long as they appear healthy and ideally with a sample of your water to test, within a few days.
You might try having a friend call them and ask if they ever take returns from customers, or buy fish from customers, about something really innocuous like a guppy or a platy, and see if they give the same answer, or if they say "sure!"
If they say they'll take something peaceful, it's probably a sign they played you for a sucker and KNEW the bad temper of the fish before selling it and were just trying to get rid of it to the first taker so it wasn't their problem anymore.
Where do you live, the US or the UK, or elsewhere? Depending on the answer you could possibly report them to the Better Business Bureau or your country's equivalent, as that exists solely for dealing with consumer complaints about businesses.
I WOULDN'T recommend just leaving the fish on their counter, because if they're as unethical as they sound, they might just leave the fish bagged up and let it die isntead of restocking it.
Good luck!