Manchester United co-chairman Avram Glazer is set to make more than £70m after putting five million of his shares in the club up for sale.
A deadline of 16 March has been put on the sale of the shares, which closed on the New York Stock Market on Thursday evening at $20.13.
There is no guarantee Glazer will recoup that amount but if he did, they would be worth $100.65m (£72.5m).
It would reduce Glazer’s personal shareholding to 10.2%.
The Glazer family’s combined ownership will fall from 78% to 74.9%.
The family have been controversial owners since they bought the club through a leveraged buyout in 2005 that valued the club at £790m.
When the club announced its second-quarter results last week, it confirmed its debt had risen to £455.5m.
It has been estimated that in the 16 years since the takeover, more than £1bn has left the club in finance costs to service the debt and pay dividends to the family.
A United source said Glazer’s decision did not represent a weakening of the family’s commitment to running the club.
Manchester United Supporters’ Trust welcomed the sale as a “further small step towards a more diversified ownership model”.
“Ultimately this is the Glazer family relinquishing some of its control, albeit in a relatively limited way,” MUST said.
“For those of us who want a change in ownership model, the Glazer family selling shares is the only way this can happen.
“The downside of this move is that the shares being sold have inferior voting rights and the proceeds from the sale will not go into the club.”