Is it possible to officially buy a BioNTech dose in Germany ?

Yes and no. You can order Comirnaty in Germany if you're a doctor's practice, and the deliveries will come from the central distribution facilities in each German state. Those facilities are receiving shipments from the Marburg production facility that BioNTech owns and operates. Similarly, if you were to do the same as a Belgian physician, you would receive shipments that originate in Pfizer's production facility in Belgium. It's exactly the same product though. I don't know if there's a difference in the labeling beyond specifying production site. The production lot labels you get for each dose to be added to patient vaccination records look exactly the same (other than the lot serial numbers of course).

However doctors cannot currently order Comirnaty "directly" from anywhere except from the government-controlled distribution system, which operates along the same basic distribution lines that are also used for other vaccines and medicine products, but is governed by special rules (for example the government can limit the number of doses a single doctor can order if demand outstrips supply).

And when it comes to ordering from the manufacturer, whether it's Pfizer or BioNTech, AFAIK nobody except nation states can do that. They just don't accept anyone else as customers - with very rare, but notable exceptions (there was this occurrence where the company TSMC ordered some millions of doses of Comirnaty as a proxy for the Taiwan government because some weird Chinese intervention, of which I didn't fully understand the exact mechanism how it worked, prevented Taiwan from ordering directly).