Speaking to the Official Club Website, the blues chief said: "Firstly I would like to thank Kelvin for his service while with us and wish him all the very best at Sunderland.
"The fee, which contrary to some earlier reporting, gives the Club a net return of just under £1 million.This is too great an amount to refuse for a player with just one year left on his contract.
"Funds are now available to the manager for further strengthening the squad and I know Joe has a number of irons in the fire."