Hi,
This came up last week when I was playing CE with my goddaughters, and it seems like the oblique answer in the FAQ is controversial. If Barbarian successfully makes a deal with his opponent, does he then pillage their hand? That certainly seems like a powerful disincentive to negotiate with him unless your hand is useless.
(For the record, we played that there was no pillage upon successful negotiation... perhaps self-serving since I wasn't the one playing as Barbarian, but it made sense at the time ).