Sounds like Blackmail in the new world.  I would record a conversation or email record, or both if you can...something like.  I have been considering your kind offer to "yelp me"  after I did a great job.  You want play him. ....If you post a bad review because I would not refund that which was paid, that door swings both ways.  I an in business and I have much to lose.  Slander me in public and cause me proveable loss and I will come after you with both guns blazing and use every resourse to prove your intent.  I will protect my livelihood with everything I have....and if it means jail for you I will come and visit.  When you have admission (response) that proves he tried to blackmail you....call the police or consider civil action.  Collect evidence.

Also the police can go undercover to nail him.

A third option (and this takes guts but it is good for you) is to knock on his door with a witness and video the conversation.  It could go something like.... I just would like to discuss this demand of a refund.  First off can you tell me the reasons why you think you are entitled to a refund.  is it still your intention to yelp me if I do not refund 50% of my bill.