I know I am a few years too late to answer this, and I hope it went well. maybe you can tell us how it went after a few years now..
what I wanted to say is you should never plan to get into a fight if there is a chance you can get out of it. fighting should be a last resort only because so many things could go wrong ether of you could get seriously injured or even killed.
I think martial arts is great in the Dojo or in the ring, but in real life it should be only a last resort
you live in an organized society with laws and rules, so if you are not at fault and someone is harassing you or threatening you like a wild animal, just report him to the police.
you could even record his voice while he is threatening you on the phone as evidence..
you could suggest to him that if he really thinks you owe him money and he has evidence then he can file a lawsuit against you,
and also you might threaten him that if he does not stop bothering you that you will inform the police and you will sue him, and he will loose a LOT more money from that than he thinks he owns you, not to mention times he might end up spending in jail,
also I suggest that if you two really want to fight it out, you can arrange to have a formal match at your local boxing or wrestling gym.. at least this way there will be people watching who can keep things from going out of hand,
I hope my advise helps for anyone reading this in the future,
always keep it civilized, unless you have no other choice