Patrick van Aanholt's move to Crystal Palace could bank Sunderland up to £14million.
Sunderland chiefs gave the Dutch left-back permission to speak with relegation rivals Palace after the defender handed in a transfer request saying he no longer wanted to play for the Black Cats.
The club accepted a £12million deal, which could be worth up to £14million including add-ons, and he is now in London for a medical and talks.
It will see the 26-year-old team up again with former Sunderland boss Sam Allardyce at Selhurst Park.
Allardyce, speaking at his pre-match press conference this morning, confirmed the Dutch left-back is undergoing a medical and said he is looking forward to working with him again.
He said: "He's in the process, at the moment.
"Goals and assists contribution [from Patrick] was massive [last season].
"His defending improved under me and he listened to advice. Lot of energy from him & Schluppy.
"Van Aanholt was a major saviour in our run at Sunderland last season. Major part of that success story."
Sunderland chief executive, Martin Bain said: "If a player openly states he does not want to play for our club, then it is clear that we have a decision to make.
"In this instance and in regard to this particular player, we have done what we feel is right for Sunderland, which will always be our priority."
Sunderland had earlier rejected a £7million offer from the Eagles for van Aanholt who wanted to leave.
Sunderland held out for their valuation for a man who joined them from Chelsea for just £1.5million in July 2014 after loan spells at Coventry, Newcastle, Leicester, Wigan and Vitesse Arnhem.