Sounds like you got some newb sales clerk making promises who then got his rear chewed out by a manager who actually understands the business.

As they are a chain, I would write a letter of complaint explaining exactly what you were told to the store manager and cc a copy to a regional office. Maybe they'll take pity on you, but again common sense would tell you you can't run fuel through a carb and then take it back to be sold as new.

Sorry you're out $300. That sucks.