Postbank as well as other banks offer more services than just the Girokonto bank account that you get your salary in and you use for bank transfers and stuff. While the Girokonto is usually free if you have a minimum income (which differs from bank to bank, usually around 1k EUR per month), the savings accounts are free.
Traditionally named Sparbuch, those now come with different names depending on the bank. In Postbank, they are called Sparcard and are now represented by a plastic card. You can use it to get bank statements out of a machine, and you can put money on it. I think (not sure about that) you can even transfer money onto those somehow from.
I have one of those which got converted from the traditional Sparbuch that I had as a kid, which I found years later in a drawer. It has some 5 Euros on it and I just left it there. Every once in a while they send me a statement because I can't remember the PIN.
It might be worth exploring that option with your bank. When leaving Germany, you could open one of those in your Postbank, move all your money there and close the Girokonto. It will give some very low interest, but it lets you keep the money in Germany, produce interest and it's covered by Einlagensicherung, so if the bank goes bankrupt it's protected.
In addtion, I would advise you go to go a couple of other banks as well and ask to talk to English-speaking people. Those would probably be in the city-center branches rather than in a small town. Explain your situation and tell them you are going to reopen a Girokonto with them once you are back, but you want to keep money in Germany in a way that is accessible. Maybe take a German friend or colleague if you feel the bank will take advantage of you.
Also, talk to the finance/accounting (Buchhaltung or Rechnungswesen) people or HR in your company and ask their advice.