Legal Authority to Repurpose Funds for Border Barrier Construction [Updated December 30, 2019] [open pdf - 1MB]
From the Document: "President Trump has prioritized the construction of border barriers along the U.S.-Mexico border. Over the course of negotiations for FY2019 appropriations, the Administration asked Congress to appropriate $5.7 billion to the Department of Homeland Security (DHS) for that purpose. When Congress appropriated $1.375 billion to DHS for border fencing, the President announced that his Administration would fund the construction of border barriers by repurposing funds appropriated to the Department of Defense (DOD) and transferring funds from the Department of the Treasury."
CRS Report for Congress, R45908
Congressional Research Service: https://crsreports.congress.gov/