U.S. Government Accountability Office (GAO) has gotten the constitutional law exactly backwards. It said that the "faithful execution of the law" — the Impoundment Control Act—"does not permit the president to substitute his own policy priorities for those congress has enacted into law ." Yes, it does — when it comes to foreign policy. The Constitution allocates to the president sole authority over foreign policy (short of declaring war or signing a treaty). It does not permit Congress to substitute its foreign policy preferences for those of the president.
To the extent that the statute at issue constrains the power of the president to conduct foreign policy, it is unconstitutional.
Consider the following hypothetical situation: Congress allocates funds to Cuba (or Iran or Venezuela). The president says that is inconsistent with his foreign policy and refuses to release the funds. Surely the president would be within his constitutional authority. Or consider the actual situation that former President Barack Obama created when he unilaterally made the Iran deal and sent that enemy of America billions of dollars without congressional approval. I do not recall the GAO complaining about that presidential decision, despite the reality that the Iran deal was, in effect, a treaty that should require senate approval that was never given.
Whatever one may think about the substantive merits of what President Donald Trump did or did not do with regard to the Ukrainian money— which was eventually sent without strings —he certainly had the authority to delay sending the funds. The GAO was simply wrong in alleging that he violated the law, which includes the Constitution, by doing so.
This Imperial Congress would make a shambles of our constitutional order. This outrageous Congressional overreach against constitutional government is what led AG Bill Barr to take on the task of defending the the institution of the presidency against the incredibly reckless actions of the Dems.