They are... not spending tax money? If so, that would still be the only entity given power by the US Constitution to decide what to do with it.

Which is a direct violation of the constitution. Only congress the authority to control spending and Musk has absolutely zero authority to stop any payments congress has authorized. It is a naked power grab and musk should spend the rest of his life in prison for it.

>Only congress the authority to control spending

You changed your wording to fit your argument. "To spend" became "to control spending", implicitly acknowledging that the two phrases have different meanings.

The executive branch does not have the authority to cut off congressionally appropriated spending. Congress specifically passed a law (Impoundment Control Act) to make that as explicit as could be