That'd be allright for cash and influence - it'd be a bit of a pain if you want to take some of his ships or tech, but. And you'd have to tell it what payment terms you wanted. Actually, it could work it out for both sides, you could replace the current bcs/influence points sliders at the top with:
BCs button
----|------ <--- months slider
Influence button
So, say you've got a kick-ass tech, and you want to swap it for a couple cheap techs and some cash. You select the techs on both sides, and the text is still green. You know you can milk a bit more, so you press "bcs", and it automatically adds, say, 500 bcs to the mix. If you're willing to split the payment over months, you play with the months slider - say, set it to 10 and you see 55bcs for 10 months.
If he won't trade his three techs for your cool one, you could either remove one of them, or click YOUR bcs button, to offer him some small change to sweeten the deal.
Same for influence, only no need for the month slider.
The only other thing I can think of is demands - you should still have to give a figure for that one, since seeing in advance how much you could demand sounds a bit cheesy. And repeatedly demanding their savings, or all their planets at once "just to see if it works" should eventually provoke a kick in the head.