Rob Shafir, a former Chief Executive officer at Sculptor Capital Management, and one of the hedge fund firm's largest shareholders, is opposed to its proposed sale to Rithm Capital because he views a rival bid as more attractive, according to a report by Bloomberg.

The report quotes Shafir as writing in a letter to the special committee advising Sculptor’s board, that a rival offer from a group including Boaz Weinstein, Bill Ackman, Marc Lasry and Jeff Yass “is clearly superior”.

The hedge fund firm agreed to be acquired by Rithm in July in a deal worth $639m, or $11.15 per share. The Weinstein-led group meanwhile, has upped its offer by 15 cents a share to $12.76, after its original bid was rejected by Sculptor for not showing "adequate committed funding".

In the letter, Shafir who owns 6.2% of the firm’s Class A common stock, and is the second-largest shareholder, wrote: “Your fiduciary duties require you to maximise value for Sculptor’s shareholders.”

In a new statement Sculptor said: "Notwithstanding the proposed price of $12.76 per Class A Share, the special committee cannot support a transaction that has significantly less certainty of closing than the transaction with Rithm."