Dan Och said the deal for Rithm Capital Corp. to buy Sculptor Capital Management “substantially undervalues” the hedge fund firm and “penalizes” all shareholders.

“Our group previously expressed concerns that the Board would pursue a transaction that does not reflect the full value of the Company, that would not maximize value for the benefit of all shareholders, and that further entrenches the interests of current company management,” the founder of Sculptor, which was previously named Och-Ziff, wrote in a letter to a special committee for the company that was attached to a regulatory filing.

Last month, Rithm agreed to buy Sculptor in a deal valued at about $639 million. The company had been embroiled in a legal dispute with Och over pay packages given to Sculptor’s Chief Executive Officer Jimmy Levin. The two resolved the legal dispute, and Sculptor formed a special committee to explore potential transactions.

Shares of Sculptor reversed earlier declines after the filing, climbing 0.7% to $11.01 at 1:29 p.m. in New York.