The scenario is as follows.
The evil wizard summons forth a demon to fight in combat against the party with the summon greater demon spell, which failed two saves against the wizard. The party's sorcerer successfully uses the banishment spell on the summoned demon (a glabrezu in this case, as he cast the spell at 8th level). The evil wizard, feeling unable to successfully break the sorcerer's concentration, just drops concentration on summon greater demon and instead casts some other concentration spell (in my case he cast the above spell again, but that's not relevant right now).
If the sorcerer's concentration on banishment drops at any time before its duration ends, what happens to the glabrezu? Does it come back? Do rounds where it was not present count against the 1d6 extra rounds of presence (from dropping concentration on summon greater demon)?
I would also be interested to see answers if the original demon was uncontrolled (it had succeeded its save against the wizard), and if the protective circle was in place.