One of the recurring themes of summer transfer windows when I was at the club was not getting sales done. Lucas was on the sell list every year and always ended up staying, either because we couldn't get the target fee or because he came back to pre-season and Jurgen remembered he liked him and changed his mind. Obviously back then we weren't missing out on huge money (£10m or so) and we'd perform better than we'd assumed in our forecasts and the money that came with that always balanced the books.
I'm totally OK with us pushing to get players out of the door before we make further signings. The reality is that if you find yourself in the last week of the window still needing to raise funds then you end up flogging players off on the cheap (or letting go of someone you wanted to keep). We've already committed huge money to Wirtz, Frimpong and (we assume) Kerkez. We now need to off-load before the final push, but we need to do that sooner rather than later. Only when those sales are done will we know exactly how much we can blow on a striker, and maybe the thinking is that if we raise a ton of cash then we can go after the finished article (Isak) rather than a riskier proposition like Ekitike.
For context, this is what's stopping United from signing Mbeumo. They already know they've spent speculatively, based on getting some sales income in, and they're probably only now starting to understand that they will struggle to off-load the surplus players, either at all or for the values they think they should get for them (because the offers haven't been coming in or they've been unsuitable). And with the CWC underway, none of those teams will be making any panic buys now, so we're back to a normal window and whether clubs want new players in time for pre-season.
As an aside, to contextualise what we did with forecasts, back then we usually assumed we'd go out of Europe in the first knock-out round of whatever competition we were in and that we would finish 5th (i.e. Europa League, not Champions League) plus early exits from domestic cups (we'd assume 1 home game in each competition). Our summer trading budget would be based on that (and would assume some sales) and we'd always work on the basis there'd be no business in January - i.e. we'd aim to spend the full budget in summer. If we were doing better than forecast then it would either balance the books for sales that didn't happen or it would give us some room to spend in January.