I've still yet to see a credible story about another club with concrete interest in Maguire - its all journalists speculatively putting 2+2 together and getting West Ham, etc - and the rumors of United wanting 40m for him are a laugh.
If he stays its probably not due to him wanting to fight for his place and more likely because there aren't any offers for him high enough that it makes sense for United to sell from an FFP perspective. If you assume a buying club won't pay him for than 100k per week then Maguire probably needs something like a 10m payoff from United to rip up his current two year 200k per week deal and sign a new one elsewhere. Giving up on loyalty bonuses will also be a factor. But if it costs United 10m to Maguire to sell him plus two years of 13m fee amortization will accelerate, then you're looking at something like 36m ffp figure for this year plus whatever it costs to sign a replacement. That is compared to an estimated 23m that it will cost to keep him.
I don't really understand United's FFP situation but if things are at all tight then this deal just doesn't make sense unless United can get a stupidly high fee from another club, like 30m-40m. But nobody is going to pay that for a 30-year-old defender with a lot of baggage and high wages, not in a world where you can get a similarly aged Conor Coady for 5m, Dan Burn for 11m, not to mention top young defenders from the continent like Botman or Aguerd for 25-30m.