When applying for a postdoc, the contact person is not the professor I want to work with, what shall I do? My situation is that I want to work with Prof. A, but in the advertisement another professor (B) is the contact person. Now, I need write an email to send my CV and research statement to the contact person. What shall I write in the email? I will write in the email I want to work in the institution and my previous research activity very briefly (The institution does not want the cover letter, so the email will serve as my cover letter). Then the problem is: Shall I simply write "Dear Prof. B,..."? If so, I worry that Prof. B would think I want to work with him. But if not, I would write "Dear Prof. A", which sounds weird because the email is sent to Prof. B. 

What shall I do in this situation?
It's simple: follow the instructions. If the contact email address is of prof. B, then you should address him in the greeting.

Do not make assumptions such as prof. B is recruiting postdoc for prof. A, or at least you should not bring this assumption to your conversation with prof. B, even if your assumption is correct. Many people may take it as disrespect.