There is source that says he signed on for his gross.

Grazer cannily took advantage of Universal's need to send a message of strength and viability; brand-new co-chairmen Marc Shmuger and David Linde went ahead with the deluxe deal. If Snider had not left, she would likely have kept the budget down. The key players got paid full price, with rich back ends. Denzel Washington had already gotten paid his upfront guarantee pay-or-play, so he signed on just for his gross. (He also got paid half his $20-million fee on Inside Man.) This way even when American Gangster makes money, so do gross participants Washington, Scott, Grazer and Russell Crowe. That adds up to a huge piece of the final gross going out the doorin all likelihood, some 37.5 25% of the gross. ($37.5 was Universal's all-time high, on Grinch.) The studios are all trying to not go over that 25%, which is still a hideous percentage. 

http://weblogs.variety.com/thompsononhollywood/american_gangster/index.html