When you are about to be fired or resign... not the best time to impose your terms. Exit packages are best negotiated at the beginning of the contract, when they want you, not at the (abrupt) end, when nobody does.
I don't believe they
can fire him unless they can pin security failure upon him; I would be more interested in looking into the Mail's motivation, than anything else. Were I he, then I think I would offer to resign in order to save the government's face, and do it in exchange for a good, golden egg. Clearly, his value in America has been terminated by treachery.
As for the person or persons who did leak this, as I would with Julian Assange, I'd off with their heads in true patriot manner. Oh, wait; I don't think we can do that anymore. You see how weak and silly those PC people can make your country look?
What makes interesting news in the public sphere is not the same thing as it always being in the public interest that it be published. In my view, it was either a deliberate attempt to disrupt the government for some political purpose or an attempt to proffer an alternative candidate more suited to Mr Trump's desires.