There is a buyout clause on major football contracts. It completely overrides any contract agreement in terms of duration that a player may have with a club so long as the amount can be paid by any club that wishes to force the breach in contract.

Not all of the players who forced their transfers were disrespectful to their clubs without a trigger; the clubs may have dishonored a deal or arrangement they made earlier. In some cases, the buyout clause will be met, and the new club that the player wishes to join is very willing to pay up, but their clubs make the process difficult, stalling the deal so it can fail when these things were discussed before and that was the reason the buyout clause was set.

So yes, there is a contract, but also in the contract a buyout agreement is detailed.