He will drive the C33 at Hockenheim on Friday morning.
But there have been paddock rumours in recent weeks about struggling Sauber's existing 2014 race lineup, comprising Adrian Sutil alongside Mexican Esteban Gutierrez.
Reports have said van der Garde's sponsors, including the fashion label McGregor, are offering "many millions" to Sauber if he is promoted to the race seat.
De Telegraaf newspaper quotes van der Garde as saying: "I'm just focusing entirely on the first free practice on Friday."
Van der Garde, who is married to billionaire backer Marcel Boekhoorn's daughter Denise, added that he is also ready should "the team have other things for me to do".