Unfortunately this Baseball Prospectus article on Alvarez, Boras, Hosmer and the Royals is a subscription-only article. But here are some Royals-relevant excerpts:

From the Pirates: "The Pirates are confident that the contract reached with Pedro Alvarez was agreed to and submitted to Major League Baseball in a timely fashion and properly accepted by Major League Baseball. In fact, the contract between the Kansas City Royals and Eric Hosmer, another Boras client, was submitted to the Office of the Commissioner after our contract with Pedro was submitted. Mr. Boras is apparently satisfied with the $6 million bonus that he secured for Mr. Hosmer and has not challenged the validity of that contract. Mr. Boras has been informed that if he pursues a claim that our contract with Pedro was not timely he puts Eric Hosmer's contract with Kansas City in jeopardy."

Kevin Goldstein: "This is a huge power play for so early in the process. This is sitting down at the World Series of Poker and going all-in on your first hand before the flop. Hosmer is suddenly hit by shrapnel—at risk of becoming collateral damage in a war that went from skirmish to blitzkrieg in about three seconds flat (or more literally, twelve days). Also a Scott Boras client, Hosmer got his six million and is already playing, going 3-for-6 with a pair of doubles in his first two games for Idaho Falls in the Pioneer League. But once again, we see the term "timely fashion." Not the midnight deadline, simply a timely fashion. According to multiple sources, Hosmer did not come to terms with the Royals until after the midnight deadline. He turned down $5.5 million just minutes before, and agreed to the $6 million offer after midnight. With the deadline approaching, the Royals seemingly contacted major league baseball and asked for some kind of window in which they could finish negotiations, and that request was granted. In multiple discussions with industry insiders, nobody that I spoke with had ever been through such a process themselves, but they universally believed that baseball would likely allow such a thing in some cases, since an extension window would be in the best interests of both sides in the negotiation."

"Now, back to Hosmer for a moment. If Alvarez' deal is ruled illegal on the basis of timing alone, then logic would dictate that Hosmer's deal could meet the same fate. It's important to note that Boras has not in any way contested the Hosmer contract, but the agent's tactics with his other client have put this one within the blast zone of the action. Which again takes us back to the legality of a window extension, and whether or not the Pirates received one. As an additional note, Hosmer has already signed an actual contract, has already played, and has likely already received a significant amount of money. There is no way he'd have NCAA eligibility should his deal be voided."