Every federal department has some flexibility to shift money around as their needs change. The rules for the Department of Defense are the most generous of the bunch. That extra wiggle room doesn’t go far enough to cover the huge cost of building a border wall, according to Todd Harrison, a defense budget expert at the Center For Strategic and International Studies.

“Money is appropriated by Congress for a certain purpose and with certain constraints,” Harrison said. “Once its appropriated, you have to use it for what it was appropriated for.”

Harrison said the military can move smaller amounts, in the range of several million dollars, from program to program using what is known as reprogramming authority. But Congress has to sign off on any major funding shift.

“You can’t do $25 billion,” Harrison said. “They need to use that authority every year because sometimes money winds up being in the wrong place. You wouldn’t want to be burning up much, if any, of your money on the wall.”