Back on topic...
As I understand it we have already received £150,000 which is 25% of £600,000 so we get nothing if he is sold for £600,000 or less; we only get 25% of any amount over £600,000 - e.g. if Liverpool sell him for £1m then we only get 25% of the difference between £600k and £1m = 25% of £400k = £100k
That's how it is, so let's hope they sell for much more than £600k!!!