The sale, which equates to around 2% of the club's shares, is expected to conclude next Tuesday.
It will take the ratio of shares, which have limited voting rights, floated on the US Stock Market to just above 20%.
Manchester United will not receive any proceeds from the sale.
The value is based on Wednesday's share price of $17.15 (£13.19).
The Glazer family bought United for £790 million ($1.34bn) in 2005. In 2012 the family sold 10% of their holding via a stock listing and have sold further shares in the following years.
Manchester United's American owners are set to earn about $72.89m (£56.04m) by selling more of their shares in the club on the New York Stock Exchange.