Nothing had been said yet, so even if they had said they come to an agreement verbally, complications like this can come up when nothing is signed. It was mentionned earlier how Diego and his father have a clause in their contract that says they get 15% of the fee, and it seems like Werder doesnt want to pay, and want us to do so.
I guess they have some leverage, because Juve cant really back out of this, when they already said this is done and dusted etc, Werder can hold us to ransom a little bit. But dont think it will work much, because I dont see any new club getting into this over just this.