Sed date dexterās fidemque haud impūne adulterō fore! But give your right hands and honor that the adulterer will not be unpunished.
If this is indirect discourse, why is there no accusative subject? Adulterō is either in the dative or ablative.
It’s because there is no subject. impune adultero est is an “impersonal” construction (one without a grammatical subject), lit. “it’s without punishment for the adulterer” (adultero is dative, impune is an adverb), i.e. the adulterer gets off scotfree.
So it’s indirect discourse all right, but there’s nothing to go into the accusative. All that can happen is that the verb becomes infinitive.