United have reluctantly agreed to a bid for goalkeeper Paddy Kenny.
The Republic of Ireland international is leaving following eight years with the Blades after Championship rivals Queens Park Rangers lodged a bid for £750,000 for Kenny - invoking a clause within his contract allowing him to talk to the London club now managed by Neil Warnock.
United have already moved to cover the departure and are confident of quickly strengthening with a replacement number one.
Kenny returned to action only at the end of last season after serving a nine month ban imposed following a positive drugs test during the 2009 play off semi-final. United stood by their player and extended his contract not long after the suspension was upheld by the Football Association.
Since receiving the approach United officials have spoken at length with Kenny and his representatives - pointing out how the club has stood by the player during his lengthy suspension.
"We are very disappointed and frustrated by what has happened," said Chief Executive Trevor Birch. "Paddy was offered a new three year enhanced contract which he has chosen to reject.
"Paddy has now had discussions with QPR and agreed terms. As a result we have been left with no alternative but to let him go."
Kenny joined the Blades in 2002 - originally on loan - from Bury. The deal became permanent and he went onto make 278 appearances for United and during that time played seven times for the Republic of Ireland.
United's attentions now switch to bringing in a new goalkeeper. Mr Birch added: "We now have to move on and swiftly find a replacement. That process is well under way to bring in the right player in good time for pre-season. I am very confident of finding that player."