I can confirm that the extreme right (not the point where the green zone ends, marked on the screenshot) of both influence and money sliders, on the left and right sides are the maximun credits or influence points the race has, so you cannot trade more than you have.
I have to add, that on 2 games I have played GalCiv2 the green zone never changed. Furthermore, the domestic policy sliders have a equivalent blue gradient zone that don't fill the entire slider, but in this case only a tiny portion is left blanked, so it may be a bug or something; sounds strange, though. Anyone could confirm?
Thanks in advance.