Sometimes the stores will allow you to take the betta, heal it, and then give it back when healthy again.
Ive done that in conjunction with a friend, and the fish was immediately sold because it was in such great shape. Our stipulation: Until the fish is returned, one will not fill its place.
The store also carries too many, so we managed to get them to sell a few less after making them realize that too much live stock will eventually die, just losing them money.
they really hate losing money...and really like making it... ;) Hit em where it hurts, in the wallet!