Hi,
I'm new to Bettas and have been obsessing over how to keep their fins healthy. One of the best ways to "cure" fin problems (according to all the books) is by keeping the water clean. None of the books are written by people who work for any company. IMO, the authors are unbias and that is what you want, right?
As was stated above, if he was sold with fins in a bad state, you're best off returning him. If you do decide to keep him, be sure not to cross-contaminate with other tanks. (buy a seperate syphen.)
Best of luck!