He doesn't need BvS for that. He's gotten virtually every major studio to bankroll one of his films at some point. Warner Brothers is lucky to have him, not the opposite. Although they might be re-thinking that after they bankrolled "Live By Night" and it just bombed at the box office.
The whole Ben out as Director was likely wanted by both sides based on one simple thing - his track record in that role.
According to IMDB, he has been Director of 7 productions:
2018 Witness for the Prosecution (announced)
2016 Live by Night (directed by) - a whopping $19M worldwide so far
2012 Argo - a whopping $232M worldwide but critically acclaimed
2010 The Town - a whopping $154M wordlwide
2008 Gimme Shelter (Video short)
2007 Gone Baby Gone - a whopping $34M worldwide
1993 I Killed My Lesbian Wife, Hung Her on a Meathook, and Now I Have a Three Picture Deal at Disney (Short)
The fact is, his track record as a Director (and even a Producer) is spotty when it comes to box office successes. His best successes were basically with Horror franchise film budgets for Horror franchise films returns.
While I think he is a great actor, if I were bankrolling a film I would think twice about him as a Director.