The Manchester Evening News says it has already attracted bids of over £2,000 on eBay - despite there being no proof the papers are genuine.
The listing on the site offers up a four-page contract signed by Keane, then-manager Sir Alex Ferguson and chairman Martin Edwards, and a seven-page document detailing Keane's earnings and other benefits, with "each page signed by Keane and Edwards".
The documents, with 19 July 1993 printed on them, outline that Keane would have earned bonuses of £125 per point and part of a £100,000 team bonus if United won the Premier League or £25,000 if they finished in the top four, in addition to an annual wage of £250,000 and a signing-on fee of £400,000.
Pictures of the items are also shown.
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