[Aule]

Both Sauron and the Balrogs were servants of the real dark lord Morgoth. I think that if Sauron was the Lt. of Morgoth the in fact he would be command of the Balrogs.
Whether or not this hierarchy would still exist after Morgoths exile is debatable, but it seems that there must have been a mutual understanding between Sauron and the Balrog as the Balrog did not not attack Saurons orcs. They were both corrupted Mair, so evil and of the same order.
Sauron undoubtably knew that a Balrog was in Moria because Orc scouts or messengers would have described a great power in the mines.
This Balrog was not the great captain of Balrogs which was Gothmog, but it would still be more powerfull than all the Ringwraiths which as has been said before were just corrupted men who had increased power due to the power of the ring but as has been said in a previouse post the ring only gives power proportionet to the wielders power.
Sauron being the greates of Aules people would easily beat down a Balrog