He'd agreed a contract, he then left for more money. He presumably signed to Latics for a salary he was happy with? If he didn't think it was "the going rate'" why did he sign?
People really need to stop looking at Shez like he is some kind of noble gent who loves us dearly. He is a bloke from Salford trying to make as much money for his family as possible. Nothing wrong with that but stop trying to blame other people for him leaving. It was his doing, his choice and he went for more cash in his pocket.