Are the
Nazgul slaves to
the Ring or
Sauron? If the
Balrog had the Ring, and used it, would the Nazgul not follow his will? I guess it's hard to say, especially since there is such a fine line between Sauron and the Ring, anyway. In
Letters ,
Tolkien says that if
Frodo had the chance to ecounter the Nazgul at Orodruin, after he had claimed the Ring as his own, the Nazgul would not be able to take the Ring from him, but would have beckoned him come forth to see his new domain and then tricked him into going to Barud-dur, where Sauron, the true Master of the Ring, would have taken it. Maybe the same would have happened with the Balrog, if it had aquired the Ring. But, possibly, almost being Sauron's equal, and having the Ring, the Balrog would have triumphed. But Sauron would live on in the Ring.
But I don't think the Balrog even knew the Ring was within his reach. He just got peeed off at
Gandalf for waking him up. Alas for the Balrog. He could have got Gandalf and conquered ME if he had know about the Ring. But he just got his butt kicked.