If you weren't billed, then no money was taken. So nothing needs to be returned.
Are are you saying you were billed despite what the Apple website claims?
You can't be both billed and not billed at the same time.
Are you certain you were billed? Does the Mac charge appear on your account?
As far as Apple system is concerned, you have not been billed, so there's nothing else for Apple to do.
If you were billed, but somehow it was not registered in the system, then you will need to contact Apple Support directly and work it out with them. Be prepared to provide proof of the actual billing from your account.
Contact an Apple Specialist at 1‑800‑MY‑APPLE.