The transaction doesn't include B. Riley's roughly 190 independent advisors or its 90 tax professionals.
The deal could help B. Riley bolster its balance sheet as it wrestles with a significant debt load.
Ackman is planning a stock-market listing for Pershing Square as soon as late 2025, sources have said.
The CEO also said it's OK for a bank to fail if contagion to other lenders can be prevented.
The investigation found that the brokerage had deficiencies in many parts of its operations.
“We should never question the creditworthiness of the US government," he said.
Top managers have been asked to identify potential cost-reduction targets.
The move builds on the existing credit-card partnership between the two companies.
The tone contrasts with Friday's jobs report showing bigger-than-expected payroll gains.
The Russian invasion of Ukraine and the Federal Reserve's policy have created storm clouds, Dimon said.