Did Saruman Believe He Could Master The One Ring?

[Raynor]

I think that Gandalf would make a great error of judgement to believe that the ringbearer would hold a higher status than the ring in Sauron's mind, so as to not even mention the ring, but mention the ringbearer. If Gandalf genuinely believed Sauron expected to find the ring in Isengard, he would definitely have said something like "He has no ring to give, he has no captive to send. He has no Stone to see with, and cannot answer the summons". To underline, if even necessary, the importance of the ring to Sauron, compared to anything else, I will give these two quotes:

Originally Posted by The shadow of the past, FotR

So he is seeking it, seeking it, and all his thought is bent on it. It is his great hope and our great fear

Originally Posted by The last debate, RotK

If he regains it, your valour is vain, and his victory will be swift and complete: so complete that none can foresee the end of it while this world lasts. If it is destroyed, then he will fall; and his fall will be so low that none can foresee his arising ever again.

With all this emphasis on the ring, why wouldn't Sauron send all his nazgul, why only one, when all his victory would be at hand?