Just a comment on this - I agree that what you say is generally true. In some approaches to D&D (eg 4e's player-authored quests) the GM might be expected to cede a bit of this sort of control also. Though not all of it, or even most of it!
I have to admit that the notion of the DM losing agency is just bizarre. How could the DM ever actually lose agency in the game? And, even if the DM did lose agency, who cares? Agency is a player facing element. If I'm DMing, I have an infinite number of NPC's. I have complete control over 99.9% of the game. Allowing the mechanics to inform how I role play an NPC is in no way actually giving up any real authority in the game.
In my view, I'd go with a fairly simple system. Each character makes a declaration of some sort for their goal - in the example, the PC wants a discount and the NPC doesn't really want to give it. You have a pool of "Social Power" that is depleted through the discussion. So, it looks something like this:
Player: I want to get a discount from the merchant. So, I ask for a discount.
DM: Ok, roll your (We'll use the 5e rules for a moment) Persuasion vs his Insight and he rolls his Persuasion vs your Insight.
Dice are rolled Ok, you both succeed, so, you each lose 2 Social Power points. What do you say?
Player: (narrating his success) Ho fat merchant. Your wares are very nice. I have just saved the city, so, howzabout a bit of a discount?
DM: (also narrating a success) Oh, sir, but, I am just a humble merchant. If I give you a discount, I take the food from the mouths of my children.
New round, checks are rolled, this time the merchant succeeds and the PC fails
DM: You wouldn't steal the bread from the mouths of my fourteen children would you?
New round, checks are rolled, PC succeeds and merchant fails - merchant runs out of Social Power POints, so, the PC wins overall.
Player: Fourteen? Last time I was here, you only had seven children and that was only three months ago. Hey, are you trying to scam me?
DM: Ok, ok, yes, yes. Ten percent for the Hero of "Insert town name here"?
Dice provide direction, players provide script.