For any business, when transfering assets or buying and selling between subsidiaries, this should be done at an arm's legnth price. Either this will be the market price (if there is an obvious market price, e.g. if Udinese sold us a team bus, it would have to be accounted for at the price of second-hand buses) or at a "transfer price" (by which I mean transfering any asset, not just a footballer), which may be estimated by accountants or economists or tax specialists. However, it is not clear what the price of a promising young player should be, and I can certainly imagine that they could try it on.
It would be up to the Italian tax authorities to challenge (as it would be the Italian tax payer losing out) and they would have to try to prove the transfer price to us was too low. Lots of multi-national firms get into trouble with the taxman trying to shift assets around the world in this way, and sometimes even come to agreements with the tax authorities what the transfer price should be before they go ahead and do it in order to avoid future legal wranglings.