If you are departing US for China, show check-in agent in US both passports. You've done your part in complying with US requirement to depart on US passport. In the unlikely event CBP raise a stink you didn't use your US passport to depart, the fault is with the airline as you presented both passports.
Show DE passport to Chinese immigration
Returning to US from China, show check-in agent in China US passport (and DE if asked).
Show DE passport to Chinese immigration
Show US passport to CBP