What is the ruling on my saying to someone, "Disbelieve if you wish," after my advising him to abandon singing and his saying to me, "Do not make me disbelieve"? And what are the consequences for me?

You did well in advising him not to listen to music, but you erred in saying, "disbelieve if you wish," as this is impermissible. Even if you did not intend to command him to disbelieve, that word might push him to utter disbelief, and thus you would have caused a greater evil than listening to music. We advise you to be gentle and wise in forbidding evil and enjoining good.