Presuming a player is currently under contract to a club, that club have the right to enforce that contract for it's duration.\n\nShould another club want that player they can offer to buy him from his current club - this offer will normally take the form of a cash bid, but the exact nature of it is entirely up to the clubs concerned. The club can then decide to accept or reject that bid.\n\nShould they accept the bid and decide they want to sell the player, it is then up to the player what they want to do. As they have a contract with the club, they can decide to stay where they are, or they can decide to move.\n\nThe top players can attract bids huge bids from other teams, but the counter side is that when a player is out of contract they are free to sign a new contract with any club they like.\n\nFor this reason most clubs try to prevent their players reaching the last year or 2 of their contract, and if a player won't sign a new one by this stage they will seek to sell him to another club while they still have some value left.\n\nThis is just an overview, and many more complex rules exist particularly in relation to younger players, and when transfers can actually take place.