I am being threatened to be sued by Australian Writer

. I am Software Engineer,who also works as a freelancer online. Recently, i took a project from an Australian Author, relating to the development of his website for his latest e-book. I delievered the entire project on July 22, 2 days after his deadline of July 20 due to some enhanced requirements from his side. He happily paid for half of work on July 20, but suddenly started threatening to sue me from Australia if i didn't returned the money back to him. Also, he moved to second company after July 22 & now he is demanding refund or saying these lines: "I have my lawyers ready to take you to court. We have your identity details. This is not a threat. In Australia we actually file law suits. THIS IS REAL LIFE." He is asking to file a lawsuit of $30,000 due to delay of 2 days for which he has lost funding(he himself is responsible for delays) ,if i didn't returned $428, which is part of entire deal of $700,even though he is using entire code developed by me & my fellows. i don't have either resources or money to pay him $30,000 ,nor i can pay him $430, as i paid lot of developers for dev. costs. I would like to know, that whether i should consider his threat seriously & suffer loss by returning $430, or i should wait for him to take legal action as he is very affluent & rich.I don't know much about lawsuits or suing but he repeatedly threatens me for that,which has traumatized me & i am clueless. It would be great ,if you can help a normal citizen of this country by devoting few precious minutes & guide me on what to do. I would be highly thankful for your consultation on this,