Manchester united already made gross mistake when it comes to transfer market this season and it's already affecting their budget. Unless they correct the way they approach transfer market, it will continue to cost them heavily. Aside that Manchester united usually pay high for average players, they also hold on for so long when it comes to releasing their players at the appropriate time. Napoli came for Garnacho in January with a decent price but they turn the offer down.
PSG also offered a very good price for Rashford some years back but they rejected the offer and no team shows interest any longer and even if they do, the price will not be reasonable again as it was that time. Same thing is happening now regarding Bruno but they don't seem to have interest to release him. Unless they learn to release their players when a big offer comes, they will continue to be in financial constraints.
Yes, the point is, Manchester United's management team is simply not performing well. If they could perform well, they wouldn't have made Manchester United what it is today. Manchester United is a financially wealthy team, but if things continue like this, Manchester United could go bankrupt. They continue to waste money on players, with no results. Furthermore, they're also inept at making a profit. As you said, Manchester United should sell players offered high prices, especially Rashford. Manchester United refused to sell Rashford when an offer was made, but oddly enough, even though they kept him, Rashford hasn't gotten much playing time.
So, I'm truly puzzled by Manchester United's management team. Why do they consistently make the wrong decisions and consistently fail to recruit players? If this continues, I think Manchester United should replace their entire management team and replace them with new, better people. It's pointless for Manchester United to spend so much money if their management team isn't performing well. This will only make things even more difficult for Manchester United. So the bottom line is that Manchester United need to make a major overhaul now and they need to start from scratch again.