Broadly speaking, countries are sovereign and can do almost anything they’d like in this context. It is most of the time limited by treaties and trade agreements, so I suppose Russia could make a stink and complain with things like the WTO.

You’d have to dig into the specifics of German law in this case, but this is really unsurprising. Sales are routinely blocked and companies nationalised (with compensation, in civilised countries, though the fraud aspect could change this in this instance).

Gazprom Germania can believe the government is wrong and take it to the courts, but the legal argument seems sound.