Max Vartan's way out of his dilemma

So, if Max is doubting his wife to be a german spy AND doubts that his own superios may test or manipulate him in some way at once i have an idea out of this dilemma. The proof of the spy-activity is the special message, wich will be adressed to Max via phone, written down by him on a block an presumably aired by Marianne and finally dechiffred by his superiors. It is a faked message.
Imagine that: Max receives the message. He know the wording, but keep it in mind an writes down on his block a fake message made up by his own. For example the fake message is red herring and he writes down blue harvest. When he check the decoded messages and finds blue harvest, he and only he would know that his wonderfull wife Marianne is indeed a spy and can think about it how to deal with it on his own. If he finds red herring under the dechipherd ones, he will know that the message is put there by someone else, his doubt that he is manipulated and tested may be verified.
Wouldn't that be clever?