Considering that Haaland has already played for Borussia in the Champions League this season, he will not be able to help his new club even if he is bought in this transfer window, so I think there is no point in buying Haaland now. This will only lead to a large overpayment (both in the player's price and in the payment of his salary). Although it may be clubs like Barcelona or Newcastle would be happy to use him to improve their affairs in the domestic championships.
This is an important point: Haaland would not be allowed to play in the CL for his new club if he were to transfer in the winter. Another important point is: Why should Dortmund let him go in the winter? They would have no advantage from it. They would get the fixed transfer fee of 75 million either way, no matter if he would leave in the winter or in the summer. And they would let go an important player they need to qualify for the CL next season.