The Treason of Isengard: Eru's Plan?

I wondered whether Eru gave the Istari a choice of free will to do and command as they wished, or were they directly under his command? If so, why was Saruman then overcome by desire to control the power of the One Ring, if he was originally sent to oppose those who harness it? I thought the Istari were meant to show through the power of Eru to some extent, but not in designs of war, but over commanding the will of the free people of the world against Sauron.

So if Eru's greatest servants also can be corrupted by the Ring, was the Treason of Isengard already planned by Eru, or does he have no direct influence on the fate of ME? Sending Gandalf alone would surely have been of greater benefit to ME than Saruman also, that is, unless Eru was not aware of what evils the Istari may turn to thereafter.