Can you explain how they owe him the money?
Unless I'm missing something, he is asking for the sum of the difference in pay between his contract with us and his new one with West Ham over the next two years.
If I'm correct in that, then he isn't "due" this money at all. He is due the money if he fulfils his contract and stays the next to years. If all parties agree to a transfer (as they must in this scenario) then a kind of break clause is activated in a contract so we can sell and he can leave.
He is essentially playing an extra card here, saying he will only go if paid off. He can try and negotiate this, but we are not obliged to accept and he isn't owed it at all.
Happy to be proven wrong.