This is a "negative proof":
The
German
word for an officer's commission in use in the period of interest was "Offizierspatent". If I search for "Offizierspatent kaufen" (i.e. "buy a commission") then I get plenty of references to the
British
army. But I haven't found any reference to commissions being openly sold in any German states. Draw your own conclusions!
Of course, this did not not rule out "Vitamin C" (connections), favoritism, opportune political decisions, and "you promote my boy and I'll promote yours" wheeling and dealing. But at least they did not openly offer commissions at a price to anyone who had the money.
A cynic could argue that the British were merely being brutally honest about something that, in a disguised fashion, was widespread in many countries at the time.