I hadn't realized the mouse wheel worked here; that will help. Will have to give it a try.
My point is that emperors wouldn't be negotiating the nitty-gritty like that...tweaking the fine points of the deal is for the State Department to do.
It reminds me of some of the comments in the Civ4 manual about eliminating things that weren't fun and finding alternate ways to get the same effect.
If I decide on what techs I want to trade, after that playing with the sliders is:
1) taking up time I'd rather spend doing something else
2) not really enhancing game play
How about this:
Choose techs/ships/planets to trade, then have a "negotiate" button with a slider (i.e., do I want cash, influence, or some balance of the 2). You hit it, the system pops back with their offer, which you can either decline or accept.