Haller's introduction as a 62nd-minute substitute for Youssoufa Moukoko, with the game finely poised at 2-2, was met with rapturous reception at Signal Iduna Park.
The former West Ham player underwent chemotherapy after a malignant tumour was discovered in July, just days after he joined the German club from Ajax in a reported £27million deal.
Ivory Coast international Haller, who made more than 50 appearances for the Hammers between 2019 and 2021, earlier this month resumed training with head coach Edin Terzic's squad at their camp in Marbella.
England midfielder Bellingham has reportedly attracted the interest of a number of Premier League clubs despite being under contract until 2025.
He opened the scoring just before the half-hour mark with a sweet strike from the edge of the box and set up the winner for Giovanni Reyna 12 minutes from time.