This question is very broad and there are lots of ways to write negotiation scenes, so I'll tackle this question from one specific angle: How to write a negotiation scene between characters.
How do these characters know each other?
First of all, any negotiation scene needs characters to be involved, and those characters will come into your scene with pre-existing backstories and relationships with each other. Having a bunch of high-stakes discussion and dialogue between characters who don't know each other very well wouldn't be as interesting as characters who know each other too well. What is their relationship with each other? Is there any history between your characters that could complicate the negotiations? If there is, try to sprinkle it into the dialogue. Make it affect how they speak to each other.
You don't want to read this:
"I'm here to negotiate," said Janice.
"Great!" said Chloe. "Let's talk."
You want to read this:
Janice wouldn't look Chloe in the eye. "I'm here to negotiate," she said. "That's all."
"Figures," said Chloe, poisonously. "It figures you would be. Let's get this over with."
And just like that, we have lots of questions about these characters and why they're interacting like this, and what history they might have with each other. We have intrigue. We have drama. And a good dollop of drama is half the fun of a compelling negotiation scene. Don't be afraid to spoon it in!
Where is the negotiation taking place?
There are so many interesting places and settings in the world to choose from, so why do most negotiations in fiction take place with a bunch of characters sitting around a table, talking blandly at each other? Or standing around in a room holding glasses of brandy? Place the negotiation in a unique or unusual setting, and you've already won half the battle of making the scene interesting to read.
What if the negotiation takes place in the back room of a seedy pawn shop, or in a bowling alley? Or a crowded taproom full of people where the characters have to shout to be heard? Or in a graveyard under a patch of trees, huddled over an old tombstone? What if it happens at the family dinner table while their kids play obliviously in the background? Find an interesting and evocative setting for your characters to have a tense negotiation, besides just a room with a table in it, and try to match your setting to the mood of the scene. A negotiation about what to do with Grandpa's inheritance money is much more compelling when it's taking place right over his grave!
What does each character want?
The next and most obvious thing to consider when writing a negotiation is to ask what each character wants, how badly or how soon they want it, and what they will do and the lengths they will go to get it. Think about why they want to negotiate.
Specifically, you can ask yourself the following questions for each character who is involved:
Why does this character want what they want? What are their motivations or ulterior reasons for that desire, and are there any deeper character-driven or backstory reasons for it? What if their real motivation isn't obvious at first, but bubbles to the surface in the negotiation scene to cause some exciting drama? A character who just wants something for no reason, or because they're just "greedy" or "evil," isn't interesting or sympathetic to the reader.
- If they want money, why do they want money? Are they trying to feed their family, pay off debts, buy their way to a new life, fund a relative's medical bills? Are they trying to fleece the other character blind, or trick them out of more money than they should pay? Is the money ill-gotten? Try to make their motivations more complex than just dollar signs.
How much are they willing to compromise? The character (presumably) agreed to meet the other character for this negotiation, so clearly they are willing to compromise on some level to get what they want. How much will they do so? What are they willing to give up, and what will they keep? Do they have any "non-negotiables?"
Will they, in the end, get everything they want? Do they come away from the negotiation satisfied? If not, do they do anything about it? How do they react to being forced into a bad position in the deal? Perhaps their discontent will set up more scenes later, or lead them to hold a grudge.
What does their negotiation style tell us about their character?
Each character will approach this negotiation in a way that tells you more about who they are. Think about how they negotiate.
- aggressive and confrontational, with lots of shouting, anger and righteous indignation?
- logical and calm, rational and reasonable, with a good deal of factual evidence and logos?
- emotional and manipulative, using a lot of pathos and "tugging heartstrings" in their arguments?
- soft-spoken and nervous, and not really prepared for a serious negotiation like this?
- agenda-driven and strawmanning, constantly bringing their beliefs or politics into the discussion and trying to derail it with ad hominem or attacks on character?
Each of these negotiation styles will make their voice distinct in the discussion, and convey important information about their character and personality.
Does everything go as planned?
In the real world, an ideal negotiation usually ends when both parties involved have compromised as much as they are able and talked things out, and are (somewhat) satisfied with the outcome. But your own negotiation scene doesn't have to play out like our boring real world, and if you just write one big long talky scene without any action, the reader's eyes might glaze over. If you feel like things are getting too dry and conversation-heavy, spice things up a little with some more drama!
What goes wrong? What is unexpected? Is there any interesting plot information revealed? Does this negotiation scene lead straight into a pulse-pounding action scene, or some gripping character drama, or some other high point in the story that will re-invest the reader? Is it interrupted by some shocking news or another character barging in? Does a calm and collected negotiation devolve into an all-out fight, or a screaming match? What happens next?
Some ways you could build drama in a scene like this include:
- One or both of the characters don't want to be here, or are deliberately throwing a wrench into the works. Maybe these are two characters who aren't usually on speaking terms, but they've been forced into this negotiation due to a mutual interest and now they have to deal with it. Maybe one of them knows a secret and they are insinuating it into their words to see if the other character will take the bait. Some interesting drama or character relationships will help to add some flavor to your interactions and dialogue.
- A plot development derails the negotiation or changes its terms. Maybe it is suddenly revealed that one of the participants has a compelling conflict of interest. Maybe one of them was being deceitful and trying to trick the other character into a bad deal. Whatever the case, something drastic happens and now a seemingly uneventful negotiation scene is framed in a new light.
- The negotiation goes badly awry. Maybe things devolve into a full-on fight and the characters start throwing fisticuffs. Maybe one of them pulls a gun. Maybe one of them betrays the other! Why not give an otherwise somewhat-dry scene a real showstopper of an ending?
In short... (tl:dr;)
A good negotiation scene has character relationships, drama, intrigue, mutual interests and compromises, and an interesting outcome and consequences. It has an interesting setting, and complicated characters who want different things for their own reasons. It might not go as planned, and it might lead to further plot developments or play some other important role in the story. All of these elements together should help you to build a framework for writing this kind of scene effectively.