Thanks for the awnsers.
But, for example, I'm playing with Spain. The moment I take Zaragoza, Valencia or Cordoba, Milan and Sicily go for my ass ASAP. I think it's very stupid how the AI works in this case. Anyways, I can deffend myself pretty good. After some battles, they open diplomatics relashioships with me (I guess the pope has something to do) and offer me 'Ceaser Fire'. Of course I want some money. I usually take 5000+ plus Trade Rights. After 15+ turns they attack me again and I take their florins one more time... AI sucks here.
So, do you think that with a 10 influence diplomat I can get more florins from this sinners?

or perhaps it only works when a deal is 'balanced' and the AI may not take it unless it's talking to this charismatic and well-knowledge man.
I will be doing some testing on this (don't know if happens only in VH campaingn). I gladly accept more points of view!