20m£ seems a little optimistic though it's not unheard of for keepers. I don't know if the price is the real hangup here though.
I'm of the opinion that if the club wants to make room and ship players that do not want to be there and are detrimental to the squad, I'd do it at a discount if it is what it takes. Somewhat contradictory with the mantra MU don't know how to sell but to me it feels recruitment is trending in the right direction and finally cleaning up wage structure, average squad age and surplus players.
Maguire I understand that the club wants at a minimum to match his remaining amortisation value. Henderson doesn't have that albatross value attached and finding a suitable backup option for cheaper if needed should be possible.
One thing I'd say for him though : Don't read too much in a single pic posted on Xeeter. (But yeah it's funny).