The problem is what the player wants. Alf Inge Haaland has reportedly been to visit RB Leipzig. They chose development when he moved to RB Salzburg over Juventus, and they might be so inclined to do it again with Leipzig who can both offer regular first team football in the Bundesliga, and champions league football.
Keita chose the same development path before joining Liverpool. The clubs are very interlinked.
I dont know if it was the bundesliga or UEFA, but someone cracked down on the transfer policies and now they are indeed forced to either loan or pay a transfer fee for the player. Since Salzburg owns the player, and the player wants, they are free to move him to Leipzig for any reasonable term they see fit involving a loan spell or a transfer fee that is market value, or in this case, buyout clause. It's the only way Leipzig can contest with the big European clubs for players of this caliber.
I personally think that there is a certain degree of plausibility that Haaland will choose Leipzig as his next destination. It really comes down to playtime. Can we offer him regular first team minutes? I don't believe he is ready, or willing to go into an environment yet where he needs to challenge for excisting spots. We have a recent nightmare scenario from Norway in Alexander Sørloth who joined Crystal Palace and bombed hard, complete waste of everyones time.
OGS being the manager at MUFC is make or break for this deal to happen. He knows the manager well and know how he will be treated if he comes here. That personal link is the only way Haaland will join Manchester United. It might not be enough, or even the right time. But without Champions League football on a rebuilding team, that is the only factor that will tip us in favor or basically any other similar team. The notion that we can't have both him and Martial in the club is.. well, what? Do we want to buy a lesser, older and slower player in case Martial gets injured, so we have to play a worse player for x time? I'm fairly certain that the two can co-exist on the same team.