If the cat has papers, its ok to say that. But be sure that the papers have her signature on them to "sign over" to the new owners.
IMO I would NOT sell the persian in your pet shop; however, I would keep the cat and place him personally. Most people on here do not encourage buying any dogs/cats from a pet shop to start. And to give you advice on selling him in the pet shop you work at, kinda goes against what we are "preaching".
And IMO the person should have done the replacing of the kitten herself. You might want to contact the breeder/cattery that is listed and let them know what happened. They might be very interested to know a kitten they sold was being placed in a pet shop and they, themselves, just might want that kitten back.
I know that if I sold a kitten to someone and they couldn't keep it, I would want the kitten back immediately - NOT rehomed to a pet shop or not knowing where my kitten was going. In fact, I had it in my contracts that if the person couldn't keep the cat, I want first choice of having it back or help place it in another home - the kitten/cat was NEVER to be placed in a pet shop or a shelter.
As far as price, up to you what you want to charge - its probably pet quality and therefore, I'd sell it for a few hunderd dollars - but find a home yourself - not in the pet shop.