Neymar would boost both his own and our commercial status if he was to sign, he would make most of that money back in a few months strictly from shirt sales alone, not to add all the fans that would come on board which would further boost finances.
Even if he was to disappear after a few years, we'd have one of the world's best players who could go on to be the best of his generation, whilst making most of that money back.
If worse comes to worse, piss Rooney off. That being said, we will never sign Neymar nor will we ever get rid of Rooney.