Let's not dismiss the Agent being a prick here either.
Something like:
Relegation he goes for 15m
Stay up then release clause 35m
That's something that id be hoping for.
They'll probably be saying at the moment nobody does anything you get nothing in 12 months so make the release fee less prohibitive if you stay up and maybe shave off 5m if you go down too.
United should be pushing thet face that he'll be getting a huge amount more in wages and be getting exposure that would open up potential opportunities elsewhere. You'd like to think all parties could avoid this being a Showstopper.
He seems to not want to uproot his young family which is a plus. No matter what he'll end up on far vetter money in the next 12 months.
Difference will probably be in 12 months. They greedy agent probably would like him to move regardless as he gets a better cut. For us, we are probably thinking that if we have a good season we'd like the chance to keep him and even renegotiate the clause upwards (or remove it) before anyone else gets a chance to trigger it.
Maybe we can be cute. If it is Marseilles then it's 30m but anyone in UK it's 40m. I don't know how bespoke these things get.