He knew Cotto was on the slide and Manny would win. It guaranteed Arum a huge payday as he promotes both guys, and it's a result for both guys, Cotto gets a career-high payday on his way to retirement and Manny gets another big win before the inevitable Mayweather loss, plus it helps build Manny-Mayweather even bigger. Arum definitely didn't make the fight because of altruistic reasons.