The year is 2012. An 18 year old Thomas Partey arrives in Spain to try his luck. He makes his debut at Leganés. After some trials, he goes to Atlético, who decide to hire him. His club at the time give him permission, but keeps 50% of his rights.
As an Atletico player, he played on loan at Mallorca and Almeria. Atletico loved him, so they contacted leganés to buy all of his rights. As the relations between the clubs were good, Leganés held on to 10% of his rights. This was in 2015.
And what happened five years later? Everything came full circle and Leganés got an easy five million. 'Mundo Deportivo' reported the whole process explained in the two previous paragraphs, which led to the sale of the player to Arsenal.
Their departure to the Premier League club was because they paid his exit clause, something that Atletico Madrid did not expect. As the figure is 50 million euros and Leganés still had 10% of their rights, they got millions.