The authority granted by Congress to the Executive Branch to move funds from one budget activity to another.
Take the Money and Run
The Constitution grants certain powers to Congress, including the power of the purse. The Executive Branch must then spend those funds as directed by Congress, often from specific accounts for specific priorities. But Congress does provide the Executive Branch some leeway in moving funds or changing priorities. The restrictions that Congress places on the Executive Branch on how to move funds vary among the 12 appropriations bills. In turn, those agencies are often required to notify the congressional appropriations committees prior to moving funds.
Can't Buy Me Love
In certain circumstances, appropriated funds can either be transferred or reprogrammed. Transferring is the shifting of funds from one account to another, either within or between agencies. Reprogramming, on the other hand, is the shifting of funds within the same account at an agency for a purpose other than what Congress specified in its appropriations. Transferring tends to have more restrictions (and is often prohibited).
Congress frequently appropriates a "lump-sum" to a large account that encompasses many federal programs with few or no restrictions. In that situation, the agency has broad discretion in how to move funds and need not formally transfer or reprogram.
In the event of declaring a national emergency, the president gains broad powers under the National Emergencies Act. The president can order emergency construction projects that are essential to national defense, such as using unobligated funds originally set aside by Congress for military construction. Both Presidents Bush and Obama used this authority to build runways, barracks, and other infrastructure in Iraq and Afghanistan. Though there is some debate about the extent of presidential powers during a national emergency, Congress can neither prevent a president from declaring a national emergency nor stop the president from reprogramming funds in this manner.
If I Had $5,700,000,000
President Trump recently signed a continuing resolution to partially reopen the government until February 15. If Congress rejects his demands to include funding for a border wall by next Friday, Trump may follow through on his threat to declare a national emergency. The White House is currently examining options for reprogramming funds to build President Trump's wall. Some Democrats have argued that this would be an illegal use of the National Emergencies Act, while some Republicans have expressed concern that this would set precedent for future Democratic presidents to bypass Congress on their funding priorities.