Many Viennese address and business directories are viewable (but not searchable) online at http://www.digital.wienbibliothek.at/pe ... einfo/5311
. I checked the 1923 edition at random, and found, in the name section, a Leo Rosenmann, banker, who was living at Gussenbauergasse 1 (http://www.digital.wienbibliothek.at/pe ... oom/171832
). Perhaps, that's your Luser. In the Branchenverzeichnis section of the same edition, there is a listing for Bank- u. Wechselhaus Leo Rosenmann, address II/1 Praterstrasse 9, tel. 42-3-32 (http://www.digital.wienbibliothek.at/pe ... iew/172682
). In the Firmenverzeichnis section, there is a listing for Bank- u. Wechselhaus Leo Rosenmann, II/1 Praterstrasse 9, Inh. Leiser (Leo) Rosenmann. Prok. Lon Zweig (http://www.digital.wienbibliothek.at/pe ... iew/172329
By examining other editions, you might find that the location or name of the bank changed, or determine a range of dates when it existed in Vienna.
Incidentally, there is also a banker Chaim Wolf Rosenmann listed in the 1923 edition.
To identify banks in Germany, I can't help, yet, but will keep this in mind should I add any German directories from the appropriate era.
Good luck and please let us know if/how the above information is useful in your research.