It would make sense for both teams to sell those players. For Barca, this sale would help them keep Messi in the team. Barca has a lot of financial issues and they need a lot of money to offer a new contract to Messi without breaking the financial control rules imposed by LaLiga. They will either need to make very important sales, or they will need to say goodbye to Messi.

Madrid, on the other hand, is looking to purchase Mbappe, who is probably the most expensive player in the world right now. They have a solid defense with Nacho, Militao, and the recently signed Alaba, so selling Varane and then using that money to help pay for Mbappe seems like a good choice.