Let's say I'm a cleric of Mystra, and I need her help, so I cast Planar Ally:
You beseech an otherworldly entity for aid. The being must be known to you: a god, a primordial, a demon prince, or some other being of cosmic power. That entity sends a celestial, an elemental, or a fiend loyal to it to aid you, making the creature appear in an unoccupied space within range. [...]
When the creature appears, it is under no compulsion to behave in any particular way. You can ask the creature to perform a service in exchange for payment, but it isn't obliged to do so. The requested task could range from simple (fly us across the chasm, or help us fight a battle) to complex (spy on our enemies, or protect us during our foray into the dungeon). You must be able to communicate with the creature to bargain for its services.
Payment can take a variety of forms. [...]
My goddess sends a Deva with siverfire wings or something like that to help me with a task, and I ask the angel to just stay still and quiet for 1 hour. Then, I start casting Planar Binding:
With this spell, you attempt to bind a celestial, an elemental, a fey, or a fiend to your service. The creature must be within range for the entire casting of the spell. [...] At the completion of the casting, the target must make a Charisma saving throw. On a failed save, it is bound to serve you for the duration. If the creature was summoned or created by another spell, that spell's duration is extended to match the duration of this spell.
A bound creature must follow your instructions to the best of its ability. [...]
Would I be able to control the angel against my goddess' will without paying the cost of the planar ally? What happens if I cast the binding at 9th level, would I get a messenger of Mystra at my service for one year, or only until the angel completes its first task?