Like Barcelona before which has problems with financial fair play for leave Messi, I think Real Madrid will have a problem with this, as I know Halland has a salary of 15 M Pounds. which should leave many players for him.
They may have the same problem with Barcelona. But I am sure Real Madrid management has planned something to avoid it. One of the ways that they may sell many players that have big salaries but have no good performance. Some of them I have said above such as Bale, Hazard, Jovic, Mariano, Modric, or other players. Bale, Hazard, and Modric deserve to be put on the transfer list because they are no longer young. While some players like Jovic and Mariano cannot show us good performance.
Remember, it is not just about being able to pay, it is also about being able to stay under salary cap. Haaland will obviously make the amount of money superstars are making right now. Even Ronaldo at his age is still making 20+ million pounds every year, that is mainly because whichever team he goes to will sell jerseys of him enough to pay it back and then some, but at the same time Haaland is not really unknown neither anymore.
This is why I believe that there is a good chance Haaland could ask 20-30 even as high as 40 maybe, that is probably his second contract on his team, but 20-30 range is definitely something he may ask and he may get. With the salary cap situation, and if they also get Mbappe, then getting Haaland on top of it will be near impossible for them.
Just two players who will cover maybe 30% of the salary cap, then with 70% you need 9 others for starters, and then at least another 8-9 players as well, that means 20 or more players for the other 70%, which is only 3-4% for each player, not going to be something simple to pull off.