While playing the English campaign there was something t hat puzzled me, mabe someone could shed some light on this.

I was allied to The Papal States, our relations were perfect thanks to some one-sided money exchanges. I was having an eye on the HRE, as they broke our alliance some years ago by laying siege to Brugge. I launched an all-out assassination warfare against them sending a team of eight top-notch assassins and six spys into his provinces. After scouting for all members of his family I defoliated their family tree truning the German provinces into Rebel settlements.
I took three of the provinces during the next few turns. As Hungary was growing stronger, me being short on funds as beating back the Mongol Invasion in the Holy Land, I decided to give the three border provinces to the Pope, cementing our alliance and putting some nice buffer states between me and Hungary.

Each time I try to give these provinces to the pope our relations drop from perfect to abmysal and he cancels our alliance in the next turn. Even when I give him all my money from the sacked cities (= 45.000) he still hates me. Why? I just quadrpled his territories and he hates me for it?

Has this something to do with the fact that I killed of the Germans? Or that I give him sacked cities?