Some contracts contain a figure that lets another club sign a player without the current club agreeing. The clause looks like a weakness but is usually a negotiated compromise.
A contract otherwise gives the club all the power
Under normal circumstances a player under contract cannot move unless his club agrees to sell, and the club can simply refuse regardless of the offer.
That leaves a player with no route to a better opportunity except refusing to play, which damages his career and his relationship with supporters.
A release clause restores an exit at a known price, which is why agents pursue it in negotiations even when a long contract is otherwise attractive.
The club buys length and certainty
In exchange for the clause, a club typically secures a longer contract than it would otherwise obtain, along with wage terms it prefers.
The figure is usually set high enough that only a serious buyer will meet it, so the club is trading a remote possibility for immediate security.
It also converts an unpredictable negotiation into a number the club has already accepted, which allows planning around a defined worst case.
The clause is a price, not a permission
When a buying club deposits the amount, the selling club's consent becomes unnecessary, but the player still has to agree personal terms.
This is why clauses are triggered far less often than they are discussed. The price may be met while the player declines the move.
Clauses frequently carry conditions too: valid only in certain windows, only for clubs outside the domestic league, or only above a particular competition level.
Legal context determines how common they are
In some jurisdictions a clause of this kind is effectively mandatory, because the law requires a stated buy-out figure for a contract to be enforceable.
Where no such requirement exists, clauses are purely a matter of negotiation and appear mainly where the player had leverage when signing.
The specific effect of any clause depends on national law and the governing body's regulations, both of which vary and change over time.
Why the figures drift upwards
Clubs set clauses relative to what they expect the market to reach, and market values have risen faster than most clauses were written to anticipate.
Renegotiation is the usual response: a club offers improved terms in exchange for raising the figure, which is why contract extensions often follow a strong season without any move occurring.
The extremely large clauses reported for elite players are generally not sale prices at all, but deterrents chosen to be unreachable.