This is presented in the middle of the story to affirm the story itself. It is as if Allah, the Exalted, is saying to Muhammad , "Or do these obstinate disbelievers say that he fabricated this and invented it himself"
(Say: "If I have fabricated it, upon me be my crimes...") This means: such sin would be mine alone.
(but I am innocent of (all) those crimes which you commit.) This story is not invented, or fabricated falsely. Because he (the Prophet) knows better the punishment of Allah for one who lies on Allah.