Promising, Will to Power, Lying

In the Genealogy of Morals II, 2, Nietzsche expresses equal contempt for one who is incompetent to keep a promise, and a deliberate false promiser, i. e. a "liar".  However, given his presentation of the Will to Power as his fundamental principle, following his noteworthy defense of the value of Lying, the conflation of the two types of Promise-breaking requires further explanation.  For, his contempt implies an evaluation that overrides the calculation that cheating another is justified if it involves an increase in Power.  An explication of that evaluation might have obviated the Machiavellian Oligarchism that continues to follow in his wake.