I recently was offered 2 researcher job offers in two different countries. The first one is from the USA and the second one is in Japan. The professor in USA prepared all the documents for a J-1 Visa and after receiving them I have been issued a visa for 2 years.
I have now been offered another, more suitable job in Japan (more related to my interest and higher salary). I am also very interested in living in Japan. Although I applied to both jobs at the same time, the selection process in Japan took a month longer than the US process.
I accepted and signed the offer letter from the USA professor. This is an agreement to the professor that I will join, moving forward the professor will issue the documents for J-1 Visa. I was also asked in the interview whether I am considering any other job positions somewhere else. I replied yes, but my first preference is this one (the USA one).
How can I politely decline or inform the US professor of my changed situation? How would the professor in the USA feel after everything is done and I decline at the last moment?