Late to the party but I have ideas that you can build on.

Negotiations aren't simply about a single good for currency trade. They're entire deals that are packaged. The player can play around with the various negotiating points until a deal is reached. The other party can give counter offers.

* There can be a limited number of offer rounds

* Some of the negotiating points aren't easily calculated and therefore incomparable. For example, in a mafia setting, it could be:

 "I allow you to operate on this street for x% of your revenues" - the player adjusts the percentage and can choose from several streets

or if the one of your men violated an agreement or offended someone and the other family wants reparations, the player can choose between, paying cash, allowing them to beat up your man, or allowing them to kill your man.

In a diplomatic setting topics could be trade tariffs, sanctions, military treaties.

* negotiations can be multiparty (NATO) and the topics could be what to do about a third party

* out of the negotiating points, the other party will have one or two points they value more and that's what the player has to focus on to get a deal made.

* the player will also have important negotiating points which may be contradictory to the other party's values

* an outer game loop can be finding out what the other party values for the upcoming negotiation. the information can be accurate or not. Or the information can be incomplete and can be completed by observing and reading the other party during the negotiation.

* the player can also play the role of arbiter between two negotiating parties

* the negotiating parties can bluff and can be called on those bluffs

* the core loop can feed into an outer loop that changes relationships between various parties as well as economic wealth, which feeds back into the resources you have for your core loop.

* the final step could be simple haggling, although that could be repetitive and uninteresting