Hi Alexander
The tax data in the material master is relevant to the departure country: in your case this is Germany.
Is the Austrian customer an external customer to your company? If yes, you can define the tax classification in the customer master (billing tab) for country Germany.
For the relevant combination for the tax condition type in VK11, define the the tax code related. For example combination: departure country (GE) x destination country (Austria) x customer tax classification x material tax classification.
In the sales order, the tax code will be determined automatically in the pricing procedure