The winger has been linked to a move to the Catalan side, with reports in Spain stating he has already reached an agreement to join Ernesto Valverde's side.
Sporting director Michael Zorc denied that a deal had been struck, while CEO Hans-Joachim Watzke has said it will take a bid of over €100 million to convince them to sell the 20-year-old France international.
However, speculation will only increase as the club were left in the dark over the attacker's current condition after he failed to turn up for work.
"Ousmane Dembele was not in training. We do not know why. We tried to reach him but were unable to," Bosz said.
"I talked to Dembele yesterday [Wednesday]. He told me he was in Paris with his friends. He did not say anything about a transfer.
"There was no sign that he would not be here. We hope nothing terrible has happened."
Dembele joined Dortmund last year from Rennes for €15m and won rave reviews for his performances, scoring 10 times in 49 matches across all competitions.
Earlier this week, Dortmund chief executive Hans-Joachim Watzke said a fee of €100m for the player was "in every case, not enough" and the Bundesliga side's asking price is reported to lie closer to €135m.
Dembele made his France debut last season and has won seven caps, scoring his maiden goal in a 3-2 friendly win over England.