Leave postdoc in the middle of contract to start work with another professor while being in one postdoctoral research I am little bit of confused. I have completed my PhD, I do not have any prior postdoc experience. Let me explain my current situation. I got a draft contract letter from lab in China (say, Prof. A) a couple of month ago. I have started the process, I am expecting the VISA within a month. However, the research field does not relate to my current research work. Day before yesterday, I got one reply from another professor (say, Prof. B), who's work directly falls in my PhD research area. This person has a very strong background. I was dreaming during my PhD research to do some work with him. Anyway, he asked me to write an proposal, he told me that the fellowship will be ensured after a couple of month. I believe that the fellowship can be granted, assuming his positive response from his side.

Now, my concerns are as follows:


I have heard that the contract in China a bit tough. It is not easy to break contract during the middle. My initial contract period is one year, then subsequently can be extended upto  three years. My plan is to go China first, then after 6 months, once the proposal is granted, I want to move to 2nd guide (Prof. B). Is it feasible? How can I make sure there will not be any official problem during the process.
It looks odd, if I want to leave any position after six months, then why should I join there? My point is, I don't want to loose current opportunity, I am uncertain about the future response and funding from Prof B. One point, the current funding in China is a huge amount almost double with respect to the funding assured by Prof. B.
Is there will be any way out like, I may go to the Prof. B with the permission  or collaboration with Prof. A. After all, everybody is looking forward collaboration. However, I am not sure about this. Is it normal scenario in postdoctoral research?
Should I discuss the matter of collaboration to Prof. B, with Prof. A, or vice-verse?
You should be as up front about the situation as you can --- with both parties. Prof. A needs to know that you are interested in a one-year deal but cannot currently commit to anything longer. Prof. B should know that you are interested but have another opportunity and will need to know about the funding. Worst case situation, you join Prof. B's team 12 months down the road. By that time he should know about the funding.

As to #1, you answered this yourself. Once you commit yourself for one year, you should stick it out. Of course it is always possible to get out of any contract, but at what cost? Seems that it would not be worth your while to antagonize Prof. A. What is not clear to me is who makes the decision to extend. If Prof. A does not want to let you go after 1 year, are you stuck?

As to #2 and 3, forget about it. Once you go to China, you will be there for 12 months, and you will work on Prof. A's project. Everything else is unrealistic.