Discontinued HU/AU 6 months before expiry of valid HU/AU

Hello forum friends, the following case design: A vehicle has 6 months HU/ AU. On the occasion of planned sale/purchase it will be presented immediately at a inspection office for HU/AU. There you do not issue new plates. Which report is valid now? The not yet expired? Or the just issued (without badge assignment) ? Which report must be shown at a traffic control- upon request? In the context the following would be interesting to know : If you buy a vehicle n wants and does with this an extended test drive , comes along on the way by a HU/ AU-examination office , then one can then let the car without consultation with the dealer / seller undergo a HU/ AU-examination ? And what happens if the above mentioned case ( no plaque assignment ) occurs ? I would be pleased about numerous opinions . Many greetings quali