The New York Times has a comprehensive look this morning at the timeline and actions surrounding the withholding of aid to Ukraine, and I strongly suggest you read it all. But this was the ultimate in political appointees (most with personal and professional ties to Mick Mulvaney) creating rationales to justify whatever Trump wanted: Just ignore the law! Like the political operatives they are, they had little interest in whether there was a valid legal basis for their arguments, they just wanted something that sounded good enough to sell, and came up with something that cemented Trump's vision of himself as a monarch, unaccountable to anyone:

By late summer, top lawyers at the budget office were developing a proposed legal justification for the hold, based in part on conversations with White House lawyers as well as the Justice Department. Their argument was that lifting the hold would undermine Mr. Trump’s negotiating position in his efforts to fight corruption in Ukraine. The president, the lawyers believed, could ignore the requirements of the Impoundment Control Act and continue to hold the aid by asserting constitutional commander in chief powers that give him authority over diplomacy. He could do so, they believed, if he determined that, based on existing circumstances, releasing the money would undermine military or diplomatic efforts.

Among other things, we also learn:

Mark Esper, Mike Pompeo and John Bolton had a previously undisclosed meeting with Trump where they tried to convince him that releasing the aid was in interests of the United States.