Forbury LionI guess it depends what the actual contract said.Mid Sussex Royal The Olise things is rumours.
I would assume if a player has a buy out clause which is fully met it nullifies the existing contract and would leave the buying club free to negotiate from scratch.
On this weeks That Peter Crouch podcast, they were covering contracts and Steve Sidwell has some of his there including a Reading one, They told the story of someone whose contract has a line about letting them talk to clubs who made a bit of £1m*, so the club let him speak to them then said we're not selling you for less than £5m* so the transfer never happened as the contract only stated they would let the player speak to those clubs, not that they had to sell for £1m*
*Can't remember the exact numbers quoted, but was along these lines
Was Charlton before our 06/07 Premier League season. Given they were poor and got relegated, he performed well in an over achieving team and got a move to Chelsea, I'd suggest he did very well out of it.