Brookfield will acquire a 62 percent stake in Oaktree in a cash and stock deal worth roughly $4.7 billion.
Brookfield Asset Management is buying the second-largest owner of U.S. malls.
Any comeback is considered to face long odds because of how far the former market giant has fallen.
He's already apologized for assaulting his ex-wife.
Institutional funds withdrew two-third of eligible cash in 2017.
The news comes after a rocky few months for Ackman, as his investment in Chipotle continues to lag.
ADP said it will file a complaint with the SEC over an 18-page letter sent by billionaire activist investor Bill Ackman.
Bill Ackman's Pershing Square is seeking five seats on ADP’s 10-member board at its annual meeting this year.
The last time Bill Ackman invested in ADP he agitated for changes, including spinning off non-core assets.