Christopher Mirasola, a law professor at the University of Houston, said that the civil servants who carried out Trump’s order to repurpose funds could theoretically be prosecuted for violating a law called the Antideficiency Act. But such prosecutions have never happened in the past, and he said in an interview he finds it difficult to believe Trump’s justice department, or even another president, would carry them out.

The ADA prohibits the U.S. federal government from entering into a contract that is not "fully funded" because doing so would obligate the government in the absence of an appropriation adequate to the needs of the contract. Accordingly, it is often cited during U.S. government shutdowns as a reason for the closure of certain departments or facilities.

https://en.wikipedia.org/wiki/Antideficiency_Act

(http://www.autoadmit.com/thread.php?thread_id=5789960&forum_id=2#49375397)