Finding Money After The House Has Been Sold At Auction

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When a house goes into foreclosure and is sold at the auction how can you find out if there is any money to be owed to the previous owner. I have been told that when a house is sold at auction, if it sales for more than what is owed the difference is the owners money. I have heard of investors that find these kind of deals and help the previous owner get their money back and then take a percentage of the money for their help. Has anyone heard of this being done? How is it done?