Via NPR:
A question of legality
The question of the legality of any attempt to change the status of USAID is connected to its origins. The agency was created in 1961 when President John F. Kennedy signed an executive order after Congress passed the Foreign Assistance Act, which mandated the creation of an independent agency to focus on development separate from politics and the military. The agency was formally established by Congress as an independent agency in 1998.
That means "it cannot just be undone, at this point, by an executive order," Konyndyk said. "To actually disestablish the agency and dissolve it into the State Department will take an act of Congress."
Source: Why does Musk want USAID 'to die'? And why did its website disappear? : Goats and Soda
I’m doubtful relying on congress is any comfort either however.