Meh, I wouldn't do a thing. Let Raiola complain as much as he wants...nothing happens in January. United likely wouldn't be able to replace him with anything of reasonable cost or quality (mostly due to their own ineptness but still). No one is going to pay a reasonable fee for him with finances being what they are. Heck, there are maybe 1-2 clubs in the world who would even consider paying his salary at its current rate. His market at his salary is effectively Real Madrid...maybe PSG if they're interested. That's it.
Similar to last year, United also has the leverage of the Euros as well. You want to be fit and in form for France in 2021? Your best chance is to find your form at United in the spring. Even if United sold him to Real, there's a fair bit of risk in moving into a new team, new coach, etc. (and Real isn't exactly lighting it up...they may not even make the Europa League knockout stage let alone the CL knockouts).
Hold him...sell him in the summer when the financial state of football is better if he's still not performing and/or wants out. There's literally nothing to gain by selling him in the middle of a season where you're playing every 4 days for 8 months and you likely aren't going to get a replacement for him.