Returning to Germany before 6 months

I have extended my visa after graduation but I have to leave Germany for a project of 12 months. I can return before 6 months to avoid expiration of the visa but, is it possible to leave again 2 weeks after without a problem. I will return to stay months before the expiration date but I want to avoid early expiration and I want to finish the 12 month project. Thank you for your information.