It is my understanding that he can only do so for 60 days without Congressional approval. After that, Congress needs to approve funding for it.
Same with the wall. All funding comes from congress. They hold the purse strings.
I know they do but in the case of conflict where does the initial money come from. The president has very broad power when it comes to national security and it seems it would be dangerous for him to have wait on Congress in the case of a threat to the country.