Dembele is currently suspended from all Dortmund activities after going on strike to force a move to Barcelona, with the German club so far turning down all offers.
The 20-year-old is seen as a big piece of Barcelona's plan to replace Neymar, who moved to Paris Saint-Germain earlier this month.
"We know where he is and have contact with him and his agent through a staff member," Zorc said.
"We will not make this transfer on the 30th or 31st of August."
Dembele moved to Dortmund last summer from Ligue 1 side Rennes.