Ajax coach Alfred Schreuder confirmed the Cote d'Ivoire international has returned to Amsterdam to regain fitness, despite undergoing treatment for a malignant tumour that was found in July.
The 28-year-old left for Dortmund ahead of the new season having finished as the Eredivisie's top scorer at Ajax, replacing Erling Haaland in Germany after the forward joined Manchester City.
While Haller is yet to put a time frame on his recovery, Schreuder provided an encouraging update on Tuesday.
"He asked us if he could come and train because he was rehabilitating here," Schreuder said, speaking ahead of Ajax's must-win Champions League clash with Liverpool.
"Then it is of course wonderful that he can be there for a while. He was able to train on a different pitch to help his recovery."
Dortmund progressed through to the Champions League knockout stages with a goalless draw against Pep Guardiola's City on Wednesday.
Haller is yet to make his debut for BVB, who sit fifth in the Bundesliga, trailing leaders Union Berlin by just four points.