How do the Planar Ally and Planar Binding spells interact with one another?

Question

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?

Answer 1

Theoretically Yes. Practically, probably not.

If the order of events you require is allowed by the DM, then this will work. But I can't really see any reason why the Planar Ally would allow this to happen. They'd either leave, attack, have Mystra intervene, or anything else the DM comes up with to prevent this.

There just isn't a good reason or method to keep them in place for that hour willingly. And you haven't come up with a way to force it unwillingly - and even then the God may intervene to prevent this.

There's also the risk that should you succeed Mystra (and the ally) are pretty miffed. Reinforcements come to aid the bound creature (aka rocks fall.)

Answer 2

You can ask, but there's no guarantee that they'll agree to it.

Spells do what they say they do, and Planar Ally says:

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.

Ask a result, you could ask it to perform the service of standing there for an hour without doing anything, but there's no guarantee that they'll agree to do so, and they may want a payment in exchange for it that might depend based on the nature and alignment of the creature summoned.

If you've done this before and forced the outside you've bound to your service to act in ways they did not like, you might need to make a significant payment, because "stand here and do nothing for an hour" is effectively "submit yourself to my Planar Binding spell". However, if you used your bound outsiders to further the agenda of those outsiders and thereby proven yourself a reliable ally, they might decide to agree to the Planar Binding spell with minimal if any payment.