Hilton Hotels Corp. would have to pay Blackstone Group LP $560 million to back out of their $20 billion buyout agreement announced earlier this week, according to regulatory filings.

The receipt of a "superior proposal" would allow Beverly Hills-based Hilton to terminate the agreement with Blackstone, Hilton said in SEC filings Thursday that included terms of the merger agreement. Should Blackstone pull out, it would have to pay Hilton $660 million.

The deal, which was announced after Tuesday's shorted trading day, send Hilton shares soaring 26 percent on Thursday, rising $9.95 to $46 in late afternoon trading on the New York Stock Exchange.

Blackstone agreed to pay $47.50 for each Hilton share, 32 percent more than the company's closing price on July 3. The purchase is a hotel industry record, and would make Blackstone the world's largest hotel operator. Blackstone already owns hotel properties in the U.S. and Europe, such as La Quinta Inns and Suites, and LXR Luxury Resorts and Hotels, a group which iincludes The Boulders Resort and Spa in Arizona and The Golden Door Spa in San Diego.

Blackstone has received commitments from a group of banks that include Bear Stearns Cos., Bank of America Corp., Goldman Sachs Group Inc., and Morgan Stanley, Hilton said, but didn't disclose the amount of debt expected to be used to finance the deal.

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