I met Professor X, during an interview at a high-ranking university in which I wasn't selected. Weeks later, the professor invited me for an informal video chat. He gave me a lot of feedback and told me that I wasn't selected after a split decision and want me to join his team at his university. He really liked my profile and said that he can help me in my career basically (in particular in developing the skills that I needed, which contributed to me not getting that job offer).
He invited me to apply for a postdoc that he's going to advertise and gave me articles to read. He didn't say "I'm definitely hiring you," but he sure gave me a lot of info about the project and really wanted me to apply. Also, at the time, I was really lost because I had 0 options and 0 interviews, and that chat gave me the first hope after months and months of looking.
Days after that chat I was invited for two interviews from two universities, A and B. I got a quick offer right after the first interview from university A, I emailed Professor X about my situation and he said you can accept and ask if a collaboration was an option. He also mentioned that his university is a lot better than University A, which is true. After some thinking, I decided to reject A's offer and wait for the X's postdoc to be advertised, which he said should be available in a few days.
Then, University B sent me an offer. Note that University B is a much better university than previous ones: top 5 in the world! I really don't know what to do! I can't keep emailing Professor X after every offer I get.
My gut tells me to honor my part of the agreement and wait for Professor X, especially that we've established that first personal contract and he has a good impression of me. Also, he also put extra effort (invited for chat, feedback, and opening a postdoc that I can apply for)
Another professor said that you don't say "no" to a high rank university, others are just surprised that I rejected an offer after months of stress and looking. Help! it's a moral dilemma. Am being unreasonable here?