Rooney, who is expected to leave United this summer, has the money invested in an image rights company, which is used like pension funds according to Football Leaks, the Dirty Business of Football, published in Germany.
In describing Image Rights Companies of top players like Rooney the book says; "Usually, the IRC are used as "cash boxes" comparable to pension funds. As long as the pros are active, they do not touch the money flowing into their image rights companies.
"Only when they finish their career can they access saved money, liquidate the companies and can transfer the balance to their private accounts against a small tax surcharge.
"Wayne Rooney, the striker from Manchester United, has in his Image Rights Company Stoneygate 48.9million pounds of tax saving money, which in the season 2014-15 rose by 2.43million pound."
The book, published by the Spiegel magazine group, claims 60 reporters, researchers and lawyers spent seven months sifting through 18.6million documents to uncover the "billionaire business" of world football.
It says: "Joe Hart, on loan to Turin from Manchester City, has in the company JCLC Promotions around £2.3million advertising money.
There is no suggestion any clubs or individuals have done anything wrong.