But, let’s look at this from the other perspective.
a.) You had a business transaction with this person. If I had entered into a business transaction with you and you called me crying and emotional, I would probably be taken aback and give myself a little space. This may be why Michael is not responding to you well.
b.) A verbal agreement is not enforceable in most cases. Your perception of what was agreed to may be different than the buyers. Too much room for interpretation.
c.) He may have had a good reason to change the name. I have a perverse wit and I can think of some twisted paths to puruse with a name of “Backstage.” Having been on the receiving end of a horse named “Horny Snowflake” I can understand the buyer’s remorse. I changed my horse’s name too.
IMHO, I would just let it go. Unless you had it in writing with a penalty clause you’re really SOL. Does it stink? Yes. Is it what you wanted? No. Is it something you’ll learn from in the future? Yes. And that is what you should take away from this situation.
You’re not going to do your reputation any good by getting into an emotional he-said, she-said over it. Add his new name to your website, do what you have to with the search engines that you come up on top in a search of his name and wish them luck on their pursuits.