The Belgium international, 26, was due at Chelsea’s Cobham training ground on Monday, following his involvement at the World Cup.
Courtois has been a Blues player since 2011, when he signed from Belgian club Genk, and was loaned to Atletico Madrid from 2011 to 2014.
In Spain a player must buy himself out of his contract.
This is usually done by depositing the amount of the buyout clause with La Liga – Spain’s equivalent of the Premier League – to release the player from his contract. The league then passes the money to the selling club.
Kepa is Spain’s number two behind Manchester United’s David de Gea and has one international cap.
He has spent the past two seasons in Bilbao’s first team, making 53 La Liga appearances.