Haller was ruled out for several months in July after Dortmund said he would have to undergo chemotherapy following his €31 million move from Ajax.
On Monday Ajax manager Alfred Schreuder revealed Haller had asked to train with his former club and is continuing his rehabilitation with the Eredivisie side.
"He asked us if he could come and train because he was rehabilitating here," Schreuder said.
"Then it's of course wonderful that he can be there for a while. He was able to train on a different pitch to help with his recovery."
Haller scored 47 goals in 66 games for Ajax after joining from West Ham United last year and enjoyed a fine 2021-22 campaign in the Champions League with 11 goals in eight games.