Because that isn’t how the Mbappé deal worked.
He went on loan with an obligation to buy (not a ludicrous 5 year loan), deferring the majority of the outgoing cash until the following summer. Monaco were happy to wait and guarantee the serious cash the following summer as they had already raked in plenty from selling Mendy, Bernardo, Bakayoko etc. No one is in a position to wait for cash this summer post COVID so I don’t see anyone swinging a deal like that.
what about a £100million down payment on haaland and if in 5 years he hits all the target then city pay dortmund £30million more. also if he reaches 10 years at city they then get another £20million = £150million