If you cannot sell them, giving to the lfs for nothing will still be better than killing them (though obviously nicer if you can get some money for them).
Never kill a fish for no reason!!! If you can just give them to your LFS, it's much more humane. Whether you get your money back or not shouldn't matter. You should have researched the fish in the first place to save you this trouble. I know this was already said - just emphasising.
Also, before you decide to keep the tiger barbs, check out the thread on tiger barbs and cories. Just seen it, seems there can be problems even with cories, which I didn't know.
Tiger barbs should be ok if they are in a group of 7 or more if they are with 4 or more cories. Angels are a different matter and should never be kept with tiger barbs.
Yes, I know this is standard advice, Sylvia, and it's what I said in my first post, but then I just happened to see that IronMan has a post just now with aggression issues tiger barbs/cories- I think he has 14 of them, so it was a bit unexpected. I'd never heard of this before.