Honestly I would really consider returning the animal for a full refund. I know PetSmart has their 14 day return policy with living (and dead) animals.
To be honest, you probably overpaid by a heck of a lot because its a retail store. I know you want to fix it, but that shouldn't be your responsibility especially if you are the they sold it to you like this. If you keep it, chances are you will end up spending at least $100 if not a lot more to get proper medication that may or may not work.
If you want big chain stores like this to step up with their care of animals, people like you (nothing negative implied) need to tell them that it is unacceptable, and you need to return it.