the problem, I think, is that they cant have it both ways.

he's not a £50mill player yet, you are paying a premium for potential, he may be brilliant.
But that's what the sell on clause is for, if you have the sell on clause and he turns into what they think he might be then you get your money then, and rightly so.
What I think is a bit OTT is wanting both, pay a premium for potential now and then pay us again if/when he reaches it.

I don't necessarily blame them for trying to get as much and as good a deal as possible, that's their job and Id want us to do the same, but I also think its wise to tell them to feck off of they want both.