How the film got greenlit

New York Times did a piece about Zodiac and it said this about Button:

With a finished script and a $75 million budget, Mr. Fincher and Phoenix approached Sony, then invited other studios to bid. The most aggressive, Warner Brothers and Paramount, decided to team up. At the same time Paramount invited Warner Brothers to share the $150 million budget for “Benjamin Button.” So Mr. Fincher agreed to do the two movies back to back.