25% sell on would not be standard. They can have a huge sell on and a low fee, or £50m and a tiny sell on. Never ever both. The buying club has the upfront risk of the huge fee/amortisation, potential injury or failed development, whilst diluting any possible upside? Southamptons only risk is that he is better than Messi and we sell him for £250m, they then wish they’d held firm. They can avoid that risk by keeping him and giving making him their highest paid player, but all other risks back on them. They can’t avoid any and all possible risks by trying to make many millions of £ profit based on what happens 3 years after they sold him. Well they can ask, and that is their right, fair play to them.
The fee on its own is wince-inducing, but almost acceptable if we want to go a bit mad. A chunky sell on would be fine if we pay them £30m. £50m plus 25% of any future profit is a counter offer designed purely to be rejected. Be interesting to see why, and what we/they do next.