Non-partisan my behind. The president has until the end of the fiscal year (September 30th) to provide the aid (which Trump did) OR notify congress why he withheld it. The only exception to that is if the law which appropriated the aid specifies a specific timing of delivery. (sorry, GAO, your "legal opinion" is wrong.)
As for "Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the opinion said - you mean like Obama substituting his own policy priority of creating DACA for the immigration law which Congress enacted?!?