Tottenham Hotspur intend to tie Toby Alderweireld to a new contract, which would stand to overwrite his current deal that contains a release clause of £25m.
The clause, which was revealed by the website Football Leaks, has been the subject of conjecture but it is understood that it essentially protects Tottenham against the defender leaving as a Bosman free agent at the end of his existing contract in 2019.
If the 27-year-old were to remain on his current £50,000-a-week basic terms up until the summer of 2019 – which is unlikely, given the club’s desire to renew – a one-year option would then kick in until the summer of 2020.
In the summer of 2019, Alderweireld would be permitted to leave if a rival club were to pay £25m for him and complete the transfer at least 14 days before the closure of the summer window.
Alderweireld would be 30 that summer and Tottenham might feel that £25m would represent a good price for him, then. Alternatively, they would get to keep him for another year. Tottenham maintain that none of their players have buy-out clauses that are currently active.
https://www.theguardian.com/football/2016/dec/06/tottenham-toby-alderweireld-new-contract-25m-clause