(Reuters) -Billionaire investor Bill Ackman said on Monday that Pershing Square is increasing its stake in Howard Hughes Holdings by $1 billion and that the real estate developer plans to buy back shares for an additional $500 million.

Shares of Howard Hughes rose Monday but were below Ackman’s offering price. It may take more from Pershing Square to seal a deal.

Bill Ackman ’s Pershing Square Capital Management is proposing to merge its newly formed subsidiary with Howard Hughes Holdings Inc., Ackman’s latest attempt to transform Howard Hughes into an empire on par with Berkshire Hathaway.