This question is intentionally asked to see how you interact with patients. Of course, you can't take a used hearing aid, but what you can do is find out why the patient wants to return it. Is it not comfortable? Are they not using it correctly so they feel it doesn't work? Is it a cost issue? Because of your ability to interact with patients, you'll be able to let the interviewer know that you'll take the time to meet with the patient, hear their concerns and figure out a plan together.
"I have had a patient want to return a hearing aid because it was uncomfortable for them. Come to find out, the patient didn't place it correctly after they left my office. After a bit of education, we were able to get a good fit for the patient."