Haller was diagnosed with the disease a few weeks after joining Dortmund from Ajax Amsterdam in July for €36m and is yet to make his debut for his new side.
Haller is currently undergoing chemotherapy after the tumor was found to be malignant.
"I walk every day and check my body," Haller told ESPN. "I haven't lost that much after two weeks. I'm even at the same level as a few months ago. That's a good sign and I hope it stays that way. I feel very well."
The Ivory Coast international was reluctant to put a time-frame on his return to competitive football, saying: "Let's just say now is not the best time to take that step, because it will take me a few months.
"The important thing is that I don't lose too much muscle mass. I have to stay fit, and a month before I return on the field I will do some work with the ball again. Now the most important thing is that my body feels good.
"My first goal is to be back on the field, to play for [Dortmund] and score my first goal. That will be a beautiful and emotional moment."
Haller was the third top scorer in the Champions League last season with 11 goals, behind only Robert Lewandowski(13) and Karim Benzema(15).
The former Eintracht Frankfurt and West Ham United frontman struck 47 goals and provided 16 assists in 65 appearances across all competitions for Ajax after joining in January 2021, winning two Eredivisie titles and two Dutch Cups.
