Lehmann, who had tried to coax the whale into the channel on Thursday, said the mammal was not yet safe and it was crucial that it remained in open water and eventually swam out into the wider Baltic Sea.
Stephanie Gross said early on Friday that the whale was being escorted by several boats including the coast guard.
Rescuers have been unable to fit a tracker to its skin because of its poor condition, and a piece of netting is lodged in its mouth.
However, local mayor Sven Partheil-Böhnke said he was delighted that the humpback had managed to free itself and experts were optimistic that it would continue to swim north in the direction of Denmark.
Even if the whale reaches the Baltic, rescuers believe it needs to swim on into the North Sea and then the Atlantic before it reaches its natural habitat.