Edit: Sorry buddy, I completely spaced on your comment there so I had to edit this.
The negotiation would have been easier because the player really wanted our team - and had for quite awhile - which can (but not always) limit a team's leverage in negotiations.
And if the team is on good terms with the player, as Bremen was with Diego, sometimes they concede a little as a thank you gesture.
But my point before was that Diego could have gone for more if he was sold after Ronaldo, Kaka, Benzema, Zlatan, etc.