Commerzbank, which owns more than 82% of Comdirect, is in the process of taking over the entire online bank.
Petrus said in December that it owns 7.5% in Comdirect.
A stake of 90% under German law would allow Commerzbank to squeeze out remaining shareholders.
Bloomberg first reported the talks.
Reporting by Tom Sims and Hans Seidenstuecker; Editing by Sabine Wollrab and David Evans