AFAIK after paying into the system for 5 years you are eligible to make voluntary contributions and are, therefore, no longer eligible to have your contributions refunded. On the other hand you will receive a German pension (which probably won't be that much unless you retire soon ) and are probably also eligible for a Niederlassungserlaubnis.
You mean, that if I stay, work, and contribute in Germany for more than 5 years, I can go back to the U.S. and receive my social security there plus a German pension? My parents did that--got a Canadian pension + U.S. social security but they had to spend 6 months in each country.
I called the U.S.social security office once and I thought what they told me was that I could receive either U.S. social security or the German one but not both, and the German one would be less because less time working here. At time of retirement, I'd have enough credits to receive full U.S. social security without 'combining' the two.