It is not just up to BVB, you also need to convince the player and his agent to agree the move as well. The player is not a product you sell to the highest bidder any time you wanted. The player and his agent will ensure that will be the right move at the right time for his career. That is exactly the reason Haaland pick BVB over other clubs this January. The logical step for him is to replace one of the big name striker in the big clubs like Aguero in City, Lewandowski in Bayern, or Benz in Real, and that will not be next summer.