The President can only divert the Military Construction money to the Wall because of a particular Law - The National Emergencies Act of 1976.
That Law specifically authorizes those specific funds to be spent on construction at the President’s discretion in an Emergency (and empowers the President to declare that emergency).
Funds must be both legally appropriated, and legally authorized by Congress, to be legally spent.
In this case, one law appropriated the money (NDAA), while another specifically authorizes its use - but it legally both appropriated and authorized.
There is not much more in the Federal budget that could be diverted, beyond what he has already (very resourcefully) found.
Right... Know that...
But I'd like to see the heads explode and the frenzy of lawsuits that would take years to reach the SCOTUS if he said he was going to do it...
After all, it's OK for the communists to talk about "war", killing the POTUS, packing the SCOTUS, and "burning the government and the country down"...
All of which is, under the "law", insurrection...
So why not stir the pot?
After the election, say he's going to do it...
Why is a 1976 Act/law more important than the Constitution and fomenting insurrection?