#3 Nemanja Matic - David Luiz
The Serbian left Chelsea for Benfica in January 2011, with David Luiz going in the other direction. Luiz was a huge success at Estadio da Luz and Chelsea spent more than £20 million in addition to Nemanja Matic to land the Brazilian defender.
Both players had a successful stint with their respective clubs after the swap. Luiz helped Chelsea to win the Premier League, UEFA Champions League, UEFA Europa League, and FA Cup while Matic won the League title with the Portuguese giants.
It was coincident that both players left and rejoined Chelsea. Matic returned to Chelsea in 2014 and won 2 Premier League titles at Stamford Bridge, while David Luiz rejoined from Paris Saint-Germain in 2016. Both played together to win the 2016-17 Premier League title with Chelsea.