"Juventus communicates to have perfected an agreement with VfL Wolfsburg GmbH for the permanent transfer of Da Cunha Diego for an income of €15.5m
fully payable once the player signs his contract."
Thats the bottom line IMO. They've sold him with a 10 million loss to get the full amount "now" not in a year or two.
Anyone with basic background knowledge of financial reports would now that companies tend to sell their

Assets

(in football clubs players are considered our assets) to balance their books and impress their shareholders even if they needed to keep that asset.
Del neri kept saying that Diego is our best player and he praised him all summer long and even Diego said that Del Neri wanted to keep him while Marotta is the guy who wanted to sell him.
To me its clear, why would you sell your best player that even your coach wants to keep for a loss "unless" you desperately want a fast cash to balance the club's books.
Thats just my little theory on the whole thing.