The True Master.

[Boromir88]

Naw, this isn't a thread based on if Sauron was fooled in thinking someone could possibly get the Ring to Mount Doom and destroy it. But, whether he was fooled into thinking he was the Master of the Ring? Was Sauron the "Master of the Ring?" Or was it the Ring that was the Master of Sauron? And Sauron only believed he was the "Master of the Ring?"

On one side we have Sauron who is called the "Lord of the Rings," the "Master of the One." Also, the Ring tries to get back to him. It wants to get back to Sauron. When the Ring can get no more help out of a person it slips away. As it did with Gollum, and as Bilbo said it had a tendancy to slip off his finger.

Then on the other side we see in Letter 131...

While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that was existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place.

As we are all aware, that you could either destroy the ring, or claim it and master it, to defeat Sauron for good. But, what I find interesting is that all this time it's not Sauron that's the master of the Ring, but it's the Ring that's the Master.

So now we know that the Ring could still survive, yet Sauron could be destroyed. (Though it would only bring about another Dark Lord Sauron himself would be destroyed).

It's interesting that Tolkien uses the word "rapport" to describe the relationship between the Ring's powers and Sauron.
rapport as in, bond, unity, or togetherness. Not that Sauron controlled the Ring's powers, but the Ring's powers were bonded to Sauron. And without the Ring's powers "bonded" to Sauron, Sauron could no long exist (in a physical form).

So, who was the true Master? The Ring or Sauron? Or were they coequal? The Ring wanting to get back into Sauron's hands. Yet Sauron could not survive without the Ring, or by someone strong enough to claim the ring, and able to harness it's powers, and thus overthrowing Sauron.