Pix
Not sure what the question is. Halachically you prob can’t take the refund because he deserves to be paid for his work. Additionally this would be extortion which is a form of geneiva.
Also, the guy is bad news. You already did a public service by posting it online. . Why would you take it down? The guy is bad and people should know to stay away. At most you can offer that you will edit it and write that he has agreed to the original price and returned the extra you paid him. You should not remove the review though.