No, you definitely need to get the betta out of there
asap. Some stores make me so mad! :x They gave you completely bogus information. Bettas are tropical fish that WILL attack other fish with fancy fins and tails, such as your goldfish. Goldfish are cold water fish, and they should not be housed with tropical fish to begin with.
My suggestion is to get at least a 1
gal. bowl or tank, and move the betta into there. He doesn't need a filter, and you may not even want to add gravel, since that would make the bowl a lot easier to keep clean. (It's really hard to vaccuum gravel in a tiny tank like that, so what I do with my betta is weekly full water changes: put him in another container, clean out his bowl, make sure the temperature matches his old water or get him adjusted to the new temp. then add him back. I have no gravel in the bowl, just a weighted plant.)
Of course, if you can afford a bigger tank for your betta, that's better. Just be sure if you get a filtered tank that you are able to adjust the filter flow, because bettas HATE current. A 1
gal. tank without a filter won't cost much, though, and keep it in a warm spot in your house.
But again, you definitely need to get him out of the tank with the gold fish. He could very well kill it, and even if he doesn't, all that stress will definitely make your fish sick.
Good luck.