"The rules are clear and well-established. Once a prize has been announced, it cannot be withdrawn, shared or passed on to other people. That decision is final and valid for all time," they wrote in a statement.
Machado is scheduled to meet with Trump next week.
"I would really like to personally tell him that we - the Venezuelan people, because this is a prize for the Venezuelan people - would like to give it to him and share it with him," Machado told Fox News this week.
Since the US captured Venezuelan President Nicolás Maduro, Washington has, at least in the short term, chosen to support the remnants of his regime, led by interim President Delcy Rodríguez.
After his capture, Trump said that Machado neither has "sufficient support nor respect within the country to lead a democratic transition."
