Just because Congress provides funding for something in the budget does NOT mean that the President must spend it.
The President cannot spend money that has not been appropriated by Congress (although Obama did). The President also cannot divert money appropriated by Congress for a different purpose (although Obama did). The President can, however, refuse to spend money that has been appropriated by Congress (every president has).
Actually your assumption that he can “not spend” money appropriated.
That is one of the things that got Nixon in trouble. He used holdbacks to pinch people to get them to do what he wanted. That was illegal.
If the Congress says the Army needs a new Fighting Vehicle...they get it.