Congress does not give the President a blank check:
A divided three-judge panel of the 9th U.S. Circuit Court of Appeals in San Francisco agreed with a lower court ruling that prevented the government from tapping Defense Department counterdrug money to build high-priority sections of wall in Arizona, California and New Mexico.
"As for the public interest, we conclude that it is best served by respecting the Constitution's assignment of the power of the purse to Congress, and by deferring to Congress's understanding of the public interest as reflected in its repeated denial of more funding for border barrier construction," wrote Judges Michelle Friedland, a Barack Obama appointee, and Richard Clifton, a George W. Bush appointee.
Keep in mind, every time the President pulls one of these stunts, he's actually slapping Congress in the face twice. He's spending money on something they didn't want him to, and he's not spending money on something they wanted and authorized. As far as this dissenting opinion: