"We tried to reach him but couldn't", Bosz told Sky Deutschland.
However, Dortmund claimed that they have rejected a bid from the Catalan giants and insisted that since there have been no further offers for Dembele, they are not expecting him to join Barcelona this month.
"We don't know where he is".
Barcelona are keen to land a replacement for the departed Neymar as soon as possible following the £200m world-record sale of the Brazilian to Paris Saint-Germain.
According to AFP, Dembele was involved in a training ground bust up with Dortmund goalkeeper Roman Burki earlier this week and has spoken of his desire to one day play for Barca.
The 20-year-old is reported to have skipped training today to fly to Spain to negotiate a deal.
The statement also tells us that the offer "did not correspond to the player's extraordinary abilities".
He has been suspended until next week, Dortmund said.
But Liverpool manager Juergen Klopp remains adamant that the 25-year-old will stay at Anfield no matter what Barcelona is prepared to pay.
Despite Coutinho's interest in a move away, Liverpool are not expected to agree to a fee under €110m from the La Liga club.
Dortmund only signed Dembele from Rennes last summer for €15m and the Frenchman has impressed in his debut season in the Bundesliga.
Dembele was already courted by Barcelona last year, but opted for a five-year deal with Dortmund to improve his game and gain more first-team experience.