The Planar Binding spell reads:
You can attempt to compel the creature to perform a service by
describing the service and perhaps offering some sort of reward.
[…] This process can be repeated until the creature promises to
serve, until it breaks free, or until you decide to get rid of it by
means of some other spell.
If the creature promises to serve… Does it have to?
Not doing the service would mean not holding their promise, but a lot of outsiders seem like they would not really care.
Are they magically bound to hold their promise? (which seems like an enchantment effect and would brings a lot of other issues)
And if the bound outsider has to do its task, can it not try really hard?
A Creature Who Promises Must Serve
The spell lesser planar binding et. al. says, in part,
To be clear, that's one of the spell's effects: to allows attempts to compel. That's part of it's magic. Exactly how that attempt to compel is made and its results are covered by the next batch of text.
Therefore there are 4 possible outcomes for the attempt to compel:
In case 4 the attempt to compel has succeeded, and the creature, by giving its promise, shows that the attempt's succeeded. There's otherwise no way for the caster to know the attempt to compel has really succeeded; it's either die rolls at the gaming table or uncomfortable silences over lines of powdered silver in the caster's basement if the creature doesn't make it clear he's compelled. The promise makes it clear that the caster won the attempt to compel.
"Yes, Yes, I'm Totally Compelled. Sure, I Promise."
[Caster Frees Creature. Creature Eats Caster.]
"Wizards. Tasty and Wisdom's a Dump Stat!"
Once the creature's promised to serve, it must serve as the creature is compelled as the spell says. Don't worry about this being an enchantment effect, magical compulsion, or ongoing magical effect; the text mentions none of that. The creature's already used its Spell Resistance during the spell, so that's not an issue either. The spell does what it does, and if it compels without being an enchantment effect or whatever, it just does that.
That said, it is up to the DM if the creature can lie about being compelled; I'd argue the creature can lie because, seriously, man, outsiders are jerks, and if a caster's doing this he should have a decent Sense Motive skill and maybe a buddy running discern lies, too, just in case. The spell says the spell's dangerous; be prepared.
But a more generous DM might say the creature can't lie, that the magic of the spell prevents a promise from being given without it meaning the creature's compelled. That is, the promise can't be faked as the promise is required by the magic to show the attempt to compel has succeeded. Such a DM would likely mandate confirmation of the creature's promise: the caster asks, "Do you promise to perform such a service in exchange for this reward?" and anything less than the creature saying, "Yes, I promise," means it's not compelled, and that's a totally valid way to run the spell.
"You Slow-walking Me, Salt Mephit?"
"O, No, Master. I'm Just Very Careful."
The DM's planar binding escape route is the the line Impossible demands or unreasonable commands are never agreed to, which means the DM can just tell the caster a creature finds any--or perhaps even every if the DM or the creature's obstructionist--service impossible or unreasonable and say the creature just won't promise to do it. The creature then wins any attempt to compel unless the caster changes the service.
That means there just shouldn't be bound outsiders who are disagreeable to a task. The DM should have nipped that in the bud when the attempt to compel was made. It's unfair for the DM to say the creature finds the initial service as described unreasonable and have the creature go about subverting instructions after it's promised to do exactly what the caster now wants it to do.
But an outsider who later finds itself in an impossible or unreasonable position nonetheless has an out with this line:
The outsider still can't refuse to perform the service, nor can it break its promise, but lawful outsiders are often portrayed as legal geniuses and chaotic ones full-on loons, so while one clever subversive creature might say, "I'm protecting you by sealing you behind this wall of force," another might say, "I'm protecting you by eating this pudding." (Okay, maybe not that pudding.)
"Demon, If You Will..."
"I Promise. We Can Discuss Details Later. Let's Do Some Evil."
Some creatures want to be brought to the Material Plane, viewing all services as reasonable or possible, assuming they'll simply subvert instructions when services become unreasonable or impossible. Such creatures are usually those that lack the native ability to use an effect like the spell plane shift to depart their planes, and planar binding et. al. lets them satisfy their desires on the Material Plane; were such creatures able to plane shift and wanted to be on the Material Plane, they'd already be on the Material Plane.
Obviously, casters should be wary of creatures who are too willing to serve. Obviously, DMs should hint that such creatures will inevitably pervert their instructions.
Most creatures--even good creatures--want something for being bound into service and need the service specified, even if the service is something they'd do anyway. Apparently, the caster is interrupting them; they have things to do, but they'll do this for the caster in exchange for that. On the other hand, "[d]emons," for example, "view [...] summoners with a mix of hatred and thanks, for most demons lack the ability to come to the Material Plane to wreak havoc on their own," and "devils often travel to the Material Plane at the summons of evil spellcasters[; being q]uick to bargain and willing to serve mortals to assure their damnation, devils ever obey the letter of their agreements, but serve the whims of Hell foremost."
But even a willing-to-serve creature must still lose the attempt to compel. No mechanic exists to allow the creature to fail the attempt to compel voluntarily, but, as an ability check, the caster and the creature can, instead of rolling the attempt to compel, take 10. Therefore, given sufficient modifiers and an agreeable creature, it's possible to guarantee the binding of an all-too-willing creature. Whether that creature who is willingly bound deserves the caster's trust is something else entirely.
DMs should have creatures who readily agree to serve be rare lest the spell's effects become a game of hyper-legal one-upsmanship or chaotic linguistic gymnastics, and the spell's power become meaningless.