Actually, there is a special fund for POTUSes to dip into. No one knows how much is in it, how much goes into it each year, or which administration started it. And it's never audited. The fund is administered by the head of the White House Military Office (who is usually just a senior NCO from one of the services). Back in the '80s, I read a book by a former Marine Segeant-Major, who administered the fund from the Johnson administration, through the Carter administration. Amongst the purchases made by the Presidents he served were: the remodeling of LBJ's Texas ranch house, the construction of helicopter landing pads at two of Nixon's homes, and the remodeling of Jimmy Carter's mom's house, as an 'historical site'. Carter's mom was supposed to have said: "Take it while ya can, Jimmy!" To which, Carter replied: "I will momma! I will!". It would be interesting to find out what Trump has been using it for?