my guess is that the CFO said what he said so that

he can have some concrete evidence to prove that you did what he accused you of.
There are many cases when someone is fired, they will sue the company--in your case, they don't want the hassle of you suing them for wrongful termination, even though they know you did it, but it's harder to prove in court, and it will cost them alot in lawyer's fees. With your letter, then it's a no brainer that you won't sue them.
If he was to keep you employed, he wouldn't have asked you to wait a week in the first place