The Bundesliga giants have made an approach to take the Portugal international on loan for the remainder of the season.
Second-placed Real Madrid host third-placed Real Sociedad on Sunday.
The former West Ham player underwent chemotherapy after a malignant tumour was discovered in July.
Haller’s introduction as a 62nd-minute substitute for Youssoufa Moukoko was met with rapturous reception at Signal Iduna Park.