Players often run in the last 12 months of their contract to earn an entry fee and clubs must be careful not to be burned.
Manchester United and Arsenal have suffered the consequences this summer, with Ander Herrera and Aaron Ramsey joining PSG and Juventus respectively after failing to agree on extensions.
Chelsea was wise to agree to sell Eden Hazard to Real Madrid to avoid losing it for free.
And clubs will think about linking their key players to long-term contracts.
Liverpool have freed Daniel Sturridge and Alberto Moreno this summer and risk losing four stars of the first team next year.
Defenders Nathaniel Clyne and Joel Matip will be free agents, as will the hero of the Champions League Divock Origi.
James Milner is also entering the last year of his agreement and Jurgen Klopp will be thrilled to keep it.
Chelsea could lose three key stars in Callum Hudson-Odoi, Willian and Olivier Giroud.
"Hudson-Odoi was strongly tied to a move in Munich in January".
Hudson-Odoi was strongly tied to a move in Bayern Munich in January and there will be a long line of suitors in case it becomes available.
However, the international of England could appear more often next season after the release of Maurizio Sarri.
Toby Alderweireld and Christian Eriksen were both heavily tied to travels from Tottenham this summer.
But in case they remain, the Spurs pair will be available for free in 12 months.
Manchester United could be in line for a flurry of high-profile exits before next season, with Paul Pogba, Romelu Lukaku and Alexis Sanchez, all linked to away games.
David De Gea is a star to whom they desperately despair, but the Spaniard is no longer close to granting an extension and could imagine a new challenge when his agreement expires next June.
And Manchester City could also lose one of their key stars in David Silva.
The director will be 34 next summer and could move on to creating opportunities for the next generation in City.