Sorry Mr. President. You don’t get to dictate the terms of an investigation if you are part of what’s being investigated (directly or indirectly). This is true EVEN IF you feel like the investigation is unfair, and EVEN IF you technically are allowed to fire the man in charge of it. In our country, rules and norms exist to protect against tyrants who would otherwise run roughshod over the checks and balances of any lesser system, and those rules and norms MUST be adhered to.
But since our constitution is not perfect and can’t possibly account for every scenario, sometimes the right thing to do, in the spirit of maintaining a free and fair society, is simply to relinquish some of your own power–especially when that power runs headlong into a major conflict of interest and ethics. You see, Mr President: there is a difference between what is right and what is legal, or at least you would have learned that had you paid attention in civics class.