There's some mad stuff in this thread. First of all re Mbappe and who can and can't afford him. Pretty much any major club could and would sign him if they had the chance. Mbappe either now or even better, next summer, is an absolute no brainer. He's already one of the best players in the world and imo and the opinion of many people, he's going to go on and establish himself as the best player in the world and also the biggest star in world football. With 1 year on his deal how much would he cost? £100m? £150m? Whatever it is, the moment he puts pen to paper on a contract with another club, his value will increase by 25%. Any club that signs Mbappe could get 5 years service out of him and sell him for more than they paid, offsetting part of what you've paid in wages for him.
As for why we manage to sell fringe players for decent fees compared to a Utd, it has very little to do with wages because we're spending the same money as Utd on wages. For the past 3 seasons our wagebill (and now revenue) is broadly the same as Utd's, in fact it was higher than Utd's in 19/20. Not as extreme as Chelsea but we're quite opportunistic with some of our signings, particularly signings of fringe players. We don't tend to buy many fringe players but the ones we do seem to be players who have short contracts or cheap release clauses that are wanted by mid table sides. The Solanke and Davies signings probably demonstrate that the best - we picked them up cheap, knowing that we could flip them for a big profit if needed. We've sold well because we've sold players that are wanted by other sides and used the loan system well to create a market for these players.