The Haggle is a companion perk. It adds a negative-cost "item" to the target inventories of trading, effectively giving free money on the first trading interaction with that entity. The value of this "item" depends on the level of the trader. This "item" can be used in all forms of barter (including compensation for rebellious companions), but not when buying items for incoming expedition.
Parsi Trader and Diplomat James Sterling come with this perk. It is possible to use multiple companions with Haggle for barter or use them in succession, each time doing new exchange (this might be especially useful when visiting Shaman Huts).