For the love of your bettas life! no cutting!!!
I have no idea what this is, if it might be a parasite or some sort of infection, but it's a good thing you didn't try as the petstore guy suggested.
IF you could get a picture of the spot it would definitely be helpful.
Now after probably scaring you half to death... (Sorry! just it would be such a bad idea), I must confess I really am not sure what you could do for him. When in doubt with a sick fish, keeping him in clean water is the best thing. You might need to move him to a smaller quarantine tank. A plastic container, or cup, or even tupperware container left to float in the main tank is fine, and it will help him keep warm.
I hope someone else can give you better treatment options for him.