"I hear it is not supposed to be allowed " - it is allowed , but it is non-trivial. Please read this page about this change (it's for military but most of it would apply to you as well) - Home of Record or State of Legal Residence Change
While you remain out of the country, you don't have your domicile in any state (if you reset your domicile to Germany or whatever new country you move to).
Only when you move back to the States will you (likely) have a domicile here in a state...so try to get an assignment to Alaska, Florida, Nevada, South Dakota, Texas, Washington, and Wyoming (no income tax).
