If you don’t want to sell, they say “this domain is not for sale”. If you say the price is a “firm $50k”, well, it looks to me like you’re willing to sell for $50k. And I don’t think he was bullied into coming up with this offer.
It looked to be 50k, but that changed. There was never a full agreement.
You can change the price of something based on the buyer’s response. Happens all the time.
Magic the Gathering, antiques, art, domains, Instagram usernames, etc. if someone starts salivating at a price you thought was wild, you’re obviously selling for too low.