I also read yesterday that Dortmund is willing to double Haalands salary if he agrees to stay in Dortmund for another year or two after this season, but i strongly doubt that this makes him stay because i think he does not see a lot of perspective at the moment the way that Dortmund is playing. If they are really doubling the salary of Haaland then i would guess that would the other players in the squad also extremely unhappy Haaland would then get around 16 Million € per year which is more than any other player at Dortmund. It's better two have a squad of good and happy players then a squad of one superstar and the rest of the team feels neglected.
This would be a big move by Dortmund for trying to keep Haaland at the team. However, I also agree that this won't be enough for Dortmund to be able to continue with Haaland for more years. I assume that his real wish is not having the double his current salary. He would like to play at a team that can achieve titles regularly like I said. And this will prevent Dortmund from convincing him for good. And as far the salary balance, I also think that the rest of the players at Dortmund wouldn't be contented with a huge gap like that.