actually a lot of players have buyout clauses. the difference being that players who dont excel will never have their buyout clause used, so we dont hear about them, but it is fairly common. players can have clauses in their contract that say they can ask for a transfer if their team doesn't make it to the Champions league even. it is very different from how business is done in North America.
teams will negotiate the transfer fee when the player's contract is close to running out. IE: 2 years left. A player who only has a year left on their contract is considered of lesser value in the transfer fee because the team with the short contract is seen as being in a position to take it now or not get any return. it's not often that a top player becomes a FA.
it's also fairly common for teams to come to terms with the player and void a contract and give the player a bigger and longer one to keep top players on their side. so if a player has a 2 year deal making 200,000 dollars a week, a team might come back and offer them a change to a 5 year deal at 300,000+ per week.