Because Dortmund has to decide how they are going to lose him. They can either lose him now for 150M, which would represent a massive Return on their investment 18 months ago. Or they can lose him for 68M next year, with this narrative dominating the headlines for a year and all the speculation and media circus that will come with it.
On the other hand, Levy doesn't need to sell, Kane has 3 years left on his deal. He won't find anyone better on the market. And the only way that Levy sells is if it's a win for him, He isn't transactional, he is very egotistical, and competitive, he will cut his nose to spite his face, He won't sell Kane to us or any English club for that matter, even if there is a gun to his head. My Guess is if Kane starts to kick up a fuss he'd be more than happy to get Paris involved and ship him off there
Third way - he signs a new contract.
Mino will know what Real will likely pay next year. Could be 160m+, easy. Dortmund know that too.
If Dortmund want to sell this year, then Mino says, ending this contract is your idea - not ours. Pay us, big time.
So it makes sense that both sides protect their interests by making a deal.
Dortmund agree an extra 10m to Erling/Mino this year, 10m for signing, 20m on the agreed sale next year. In return, they agree to sell to Erling's preferred club, maybe tho they stipulate the bid must be within 10m of the highest offer on the table.
Mino now has the whole year to use that promise get Real to commit to a 50m signing bonus and fee. And salary. If he can show he's already on 300k p/w, it's better for him when he's negotiating with Real and others. And he's obviously more likely to get huge offers, when the COVID crunch is over and cash is flowing again. We know it's hit the Spanish clubs hardest.
Mino wants to wait until they are back in business. Also wants paying, and the upper hand in the deal with the next club. But he has this whole year to negotiate, with a cast iron guarantee that the deal can be done with Dortmund. Perfect position to be in.
Erling gets paid now, and next year, and gets his choice of club - within reason. He might well want that move to Real above all else, and that would have to wait. But his development would be well served by him staying another year at Dortmund.
Dortmund avoid having to pay Erling off and accept whatever bid there is in this depressed market this year. They can count on the sale, and know the incomings and outgoings NOW. Next summer, their position is stronger than this year - more bidders, with most cash. They also get another year out of Erling, whose price is likely to skyrocket.
I'm pretty sure this is what Dortmund do, when they hang on to players for an extra year. They make it work for them AND the player. And it allows them to plan ahead in a way most clubs can't.