We plan to purchase a Lander ticket from Munich to Salzburg. Question is, can I still use this ticket from Salzburg to Berchtesgaden and back to Salzburg that day ?
Yes, you can.
thanks!
You can with the train. There is a rail line from Freilassing to Berchtesgaden; it is entirely within Bavaria, so the Laender Ticket is valid, as it is for the short hop over the river from Salzburg to Freilassing.
There is also a slightly faster bus connection, called the Waltzmann Express, that runs along river valley that goes to the Salzach between Salzburg Hbf and Berchtesgaden Hbf The Bayern-Ticket is valid on that bus only for the part of the trip (about half) in Germany, to/from the last stop in Germany.