Im from Germany and move to China next week to start a job.

In accordence with my employer, I applied for a 90 day business M type. The plan is to leave China just for one week to get my Z type in Germany.

Unfortunately, I only got a 30 day visa because of the G20 summit. I already booked my flight, with the return flight at the beginning of december. I understand, that I can extend my M type once for another 30 days, but my booked return flight is kind of useless now.
Do I need a return ticket to get through customs or could I buy one in China? Should I just book a flight from PRC to Germany and back for the beginning of november, hoping I can extend my stay?

Thanks in advance!