POTUS Trump runs the Treasury Department. He can order them to withhold disbursement of any funding.
The courts would probably uphold a lawsuit against that, presumably claiming that his power was simply to execute the law, and that impounding the funds would be legislation by fiat.
(Of course, no such lawsuit was launched against Obozo’s numerous executive orders or W’s redirection of TARP funds to “rescue” the Big 3, as far as I know.)