It may seem backwards, but the key to adjudicating Parley is knowing if your NPCs have something to ask of the PCs.
So, let's look at some play where that's true and I'll backmask the table talk into it later.
Stringfellow's party approaches the walled outpost. It's the dead of night and pouring down rain, and the buildings in the steading are the only cover for miles. A guard leans out a high window as they approach. "Shove off, you lot, gate's closed until sunup."
Everyone else looks at Stringfellow expectantly, and the bard steps forward, clearing his throat. "Sir, please. We've traveled a long way and this is our only hope of shelter. All we want right now is a dry place to spend the night."
The guard lets out an aggrieved sigh. "Fine, fine. Lay down your weapons and I'll open the wicket gate for you. You can come get them in the morning when we get you into the city proper. Maybe if you sleep light enough you can pretend you made it in early and the captain won't go spare."
This seems like a reasonable course of fiction, right? Stringfellow asks nicely and gets the party something they wanted. This is what it looks like as a Parley, featuring the GM and Stringfellow's player, Alex:
Stringfellow's party approaches the walled outpost. It's the dead of night and pouring down rain, and the buildings in the steading are the only cover for miles. A guard leans out a high window as they approach. "Shove off, you lot, gate's closed until sunup."
GM: So are you going to turn back and try to find a dry place, or...?
Everyone else looks at Stringfellow expectantly, and the bard steps forward, clearing his throat. "Sir, please. We've traveled a long way and this is our only hope of shelter. All we want right now is a dry place to spend the night."
Alex: I think that's a Parley, right? Our leverage is... we're not going to cause trouble? Does that work?
GM: Yeah, I think this guy might be willing to bend the rules. Roll for it.
Alex: :clatter: Phew. 7, barely.
GM: Right, so he wants you to account for yourselves properly in the morning, like a bunch of people who don't want to cause trouble would do. As far as assurance of that, hmm... ah!
The guard lets out an aggrieved sigh. "Fine, fine. Lay down your weapons and I'll open the wicket gate for you. You can come get them in the morning when we get you into the city proper. Maybe if you sleep light enough you can pretend you made it in early and the captain won't go spare."
GM: Take it or leave it, folks?
Parley Backwards: "What would they ask of you, and why?"
The key here is this bit of the Parley rules:
On a 7+ they ask you for something related to whatever leverage you have.
And you can see the GM talking it through in the example - if you know what somebody would ask and the reason why they'd ask it, you have enough to rule on Parley. The reason why they'd ask it is valid PC leverage.
Sometimes that reason is just "we live in a society, and I'm acting in a civilized way"! Though of course that's a more sensible reason for someone to ask something of you in the notionally more peaceable society of Dungeon World than it is in most places in Apocalypse World.
The Whole Breakdown
So, given that Parley covers the case where a) it makes sense in the fiction that a friendly face would be able to make progress and b) the GM can come up with a promise they'd expect the PC to keep, what happens when those aren't both true?
Not A and Not B: The Dead End
This is what your "I want your magic sword" example could potentially turn into - through the principle of address the characters, not the players, the GM can just straight up say that there isn't any simple promise Sir Telric would accept in return for handing the sword over, if it's something that in-fiction would be readily apparent to the characters.
But since it is a dead end and the GM end of the conversation is supposed to present the players with something to react to, as a GM you can't just stop there. Presumably either there's something more to interest the PCs in the scene than Sir Telric's sword, or if the PCs need Sir Telric's sword for a plot reason, the GM could ask the PCs what else they might do that isn't asking, or perhaps say that leverage to get Sir Telric to give up his sword sure would be useful or valuable to you, hint hint Discern Realities hint.
Whatever that way forward is that you offer as a GM, it's probably something in your prep and you can make a move exploiting it.
B but Not A: The Unfriendly Promise
You can see an example of that in the Parley commentary:
Pendrell: This is the place where One Eye plays cards, right? Okay, I walk up to the guard. "Hey there fellows, care to, you know, open the door and let me in?" and I'm being all suave and cool so they’ll do it. Parley is roll+CHA right?
GM: Not so fast, slick. All you've done is say what you want. The big smelly one on the right steps in front of you and says, "Sorry sir, private game," all bored-sounding. It's like he hates his job and wishes he were someplace else. If you want to parley, you’re going to need some leverage. Maybe a bribe?
This is also an example of address the characters, not the players, dropping a hint that Pendrell-the-character would certainly be considering at this point in the fiction. It might not even be a matter of "present cash and roll Parley" - if the guard is fine letting Pendrell in with 50 more coin in his pocket the GM can just say that, no roll required, or the Parley roll might determine how much surety the guard demands up front.
A but Not B: Charm and Social Grace
When you act with charm and social grace despite an imminent threat, roll +CHA.
You probably recognize that as Defy Danger, chopped up and paraphrased. Maybe you're having trouble coming up with a promise exactly, but you can see how a friendly face would make progress and how it might go wrong. Maybe this just seems like a scenario where the PCs wouldn't be trying to make a promise even on a clean success. In cases like those you probably want that application of Defy Danger, where the threat is... suspicion? A sleepless night in the rain? Come up with something that makes sense to you, and that you're alright with compromising on a 7-9, and you're good.
Here's an example from the Dungeon World Guide, a third-party supplement that you can download from the main Dungeon World site here:
Fast-talking her way past two guards, the Thief rolls a 9 to Defy Danger. It's not Parley; there's no leverage. So what happens? Well, it's fundamentally a success, that’s important, so they don't arrest her. But they're not fully convinced, that's for sure. Here's some quick ideas of how the situation could go down:
- Use Up Resources: they ask for a bribe (of course they call it an "immediate payment of fine").
- Offer an Opportunity With a Cost: One of them goes off to corroborate the story; she might be able to defeat the lone guard now that he's alone, but she has to act fast.
- a Partial Success: they'll let her past, but not without a chaperone. She's where she wants to be, except there's a guard with her!