Congress spends money, not the President.
A President can (and sometimes should) make things difficult, but in the end he can’t stop the money unless he vetoes the spending bill.
Really, the President and his management of all of the various alphabet agencies isn’t part of the spending process in Washington?
I mean, when did Congress get back to deciding exactly how each dollar is spent? Because from my experience and short history on this earth, I have seen a dollar amount issued in the name of something or a department, but how it is actually spent is decided by each alphabet agency! Good example, Congress approved the Defense budget, but Trump lowered the cost of several defense weapons systems - do you think that money went back to Congress for additional spending discussions? NO!
Otherwise, we would already have a wall, since that was voted on by Congress many, MANY times.