Consider: (excerpt)
Every year funds are allocated for federal projects and programs based on estimates.
Congress adds money when those estimates fall short, even if caused by waste and fraud.
Funds sit idle when spending is less than expected. Families face this situation all the time. You take cash to buy fast food and have change left over. That change ends up in a coin jar.
A family member eventually takes the coin jar to a bank or Coinstar to turn pennies, dimes, nickels, and quarters into easier spent dollars.
Presidents, since Lyndon Johnson, have funded their priorities mining these unobligated balances through the "budget sweep" mechanism. The president authorizes the director of OMB to request executive branch departments and agencies to return unobligated and unexpended funds back to the Treasury.
LBJ funded his Great Society and the Vietnam war with the billions in "loose change" lying around executive branch agencies. Nixon funded the Vietnam War. Carter funded expanding domestic programs. Reagan brought down the Soviet empire.
The Bushes fought their Iraq wars. Clinton juggled funds to stay within Republican congressional budget limits.
Then something changed.
During his eight years in office, President Obama allowed $914.8 billion in unexpended, unobligated, funds to pile-up across the federal government. He never did a budget sweep. This number continues to climb under President Trump, who has also not authorized a budget sweep.
Unexpended, unobligated funds are dutifully reported under "Assets and Balance Sheets" of the federal budget released each year by OMB.
Under the accounting code "1941" these funds are documented — in detail — in every department and agency budget . For the current fiscal year, there is over $150 billion in "1941" unexpended, unobligated balances in the Defense Department alone.
An additional $1.028 trillion remains unexpended among general accounts, and $461 billion remains unspent in trust funds. While these funds are technically obligated, the fact that they languish for years raises questions about their use, management — and relevance.
There is, therefore, a total of $2.651 trillion in existing executive branch funds potentially available for immediate reallocation.
DC's Loose Change Could Fund Border Wall
My understanding is that Trump has directed his cabinet and other agencies to find such funds. He won't have to use authorized by unappropriated funds, the money is already there, especially in the DoD.