How to politely decline a postdoc job offer after signing the offer letter?
Joining the other commenters: I'm not sure what you think a "commitment" is if signing an offer letter and having your employer issue a visa do not qualify!
I'm not sure why you think that your answer in the interview is relevant: it is perfectly normal for you to consider other positions until you accept one, at which point you should take yourself off the market.
I think it is very likely that the offer is not legally binding: I have seen such things happen to departments, and they have never pursued the matter legally. What will happen though is that the people who offered you the job will feel that you reneged on them, and in particular that you broke your word. That they have gone to the time and expense of successfully getting you a visa is likely to amplify their feelings of ill will. (Also, it could make them be less willing to hire foreign workers needing visas in the future: please think about the implications of that for a while.) It is possible that this could damage your academic and professional reputation in the long-term with this faculty member, at this university, and perhaps in broader circles containing them. It is also possible that they will get over it rather quickly and hold no lasting grudge.
If you want to renege for a permanent academic job in a country that you distinctly prefer living in: in my experience, while some will call that action dishonorable, most will ultimately countenance it as something that you needed to do in order to get the happy life you want. But if this is a temporary job for which you cannot see your way to your next job or staying in the country you want to live in: this could turn out to be a decision that you regret. Whatever you do, I certainly urge you to get up to speed on how academic hiring works: this will stop you from making serious mistakes in the future (and may convey other advantages in your future career).
TL;DR - Politeness is the least of your worries. You may need to engage in some damage control here.
This seems a little messy to me.
That you've received a visa under the J-1 visa program means that you've got a commitment with a sponsor enough for the US government to agree to providing you with a visa. The application for a J-1 visa is rather time-consuming and will involve the outlay of resources. At the very least, the sponsor must provide evidence of insurance and fees. It is likely that these have been covered by your sponsor.
Thus, you have two problems that you need to resolve.
First, you need to inform your employer/sponsor. You need to understand that because they have already issued you with a visa, then it is highly likely that they were anticipating that you would take up the offer. You need to review the paperwork you've signed. It is likely that you have misunderstood the implications of commitment. In my university, visa processing does not begin until there is a firm commitment from the candidate in the form of a contract. Whether it is a contract or not in the legal sense is for you to determine. What is important is that the sponsor has prepared for your arrival. This means that
- They have stopped the job search, something they will need to restart
- They have planned a training or work program (a requirement of the J-1 visa) that they cannot now fill
- They may have paid fees on your behalf for visa processing, insurance, housing reservations
There are many other things that might have happened behind the scenes. You will need to appreciate all of these and understand how your refusal may cause them not a little bit of disappointment.
Indeed, if I were the sponsor, I would be furious about this development.
Your second problem is with the US government. The circumstances under which you gained the J-1 visa now no longer exist. You will need to work with your sponsor to have the visa cancelled. You do not want to travel to the US with this hanging over you as the immigration service is another world altogether.
I wish you the best of luck.
I'll be blunt, you would be losing a lot of face if you back out of the job offer in the United States a month after it was issued. It could be so far as to prevent you from ever working with anyone in that group again.
While an offer letter is not a legally binding contract, the understanding in the United States is that by signing and returning it you are accepting their offer and committing your services to them. Typically this also means you should contact other prospects to let them know that you are no longer on the market. In the event you have another interview lined up, usually people in the United States will inform the people that made the offer and negotiate accordingly.
Generally backing out of a offer letter is only acceptable in the United States if extenuating circumstances occur that would render you unable to do the job. Think something along the line of major illness, death in the family, and so forth. Even then, depending on the circumstances and the position, the employer might just negotiate a delayed start date for you to allow you to take care of things.
As mentioned by another person, double check what you signed. I find it very hard to believe that they would have started the J-1 Visa process without having an employment contract from you as well. In contrast to the offer letter, an employment contract is a legal document that has clauses enforceable in court. They can't force you to work but there may be a clause that allows them to claw back fees associated with your hiring process if you back out of the contract in bad faith.
Pragmatically, if you haven't already tried to back out of the deal with the group in the United States, I would recommend that you just politely decline the position in Japan and continue with the offer you originally accepted. Granted you would be in the United States instead of Japan, but if you can get an offer from a group in Japan once, most likely you can do it a second time.