Playing on Easy...  One empire I went to war with had -30k gold and an army that looks like it cost the full 30k.  So I entered peace treaty and negative gold into the trade menu.  He accepted it, he now has max int -30k (2 billion and change) and I'm at negative max int + 30k (-2 billion and change.)

1) I really doubt AIs should be cheating so much on easy as to go -30k gold.
2) Given what happened when you trade at negative 30k gold I doubt the game ever took into account it would be possible to do so.