The US Constitution grants Congress, not the executive branch, the power to determine how the federal government’s money is spent.
Elon Musk’s team within the Executive Office of the President have reportedly gained full admin access to the payment system of the Bureau of the Fiscal Service at the Department of the Treasury, which is responsible for the majority of the federal government’s payments, and are using that access to prevent certain payments from being made. Among other things, they are also attempting to close down the US Agency for International Development, which was established and funded as an independent agency by an act of Congress.