Actually, i tend to think that he can't release it.
In the case of Madara, Kabuto wasn't the one who summoned him. We know that. We saw who summoned him.
Muu was the individual that summoned Madara.
He even used his own chakra reserve to summon him; he specifically noted that it was going to take a little time to summon Madara because in his split form, he only had half the chakra.
Now whether the fact that Muu was the one who summoned Madara is significant or not, we don't know. Kabuto probably exerted some control over Muu, and while he was doing so he may have made it so he was capable of releasing Madara, but we don't know.
I tend to think that Kabuto was certain he'd have to face the risk of genjutsu at some point, so, i don't see why he would do that.
Anyway... just my opinion.