Invitation for campus visit for post doc position - salary negotiatons? I have had a video conference with a university research center in the U.S. The interview lasted for about an hour and the professor told me that they would like me to come to campus to visit their center. I was told about the salary and it is lower than the average post doc salary that is followed by the NIH. However, he mentioned that there is additional 20 thousands dollars (I think he said for conferences but I am not sure). 

The big advantage in this position is that it will allow me to focus on research without being bothered with heavy load of teaching. At the same time, I have been on campus interview for a tenure track faculty position and I am waiting to hear from them. The tenure track is for a position that focuses on teaching but provides me the opportunity to do my own research as much as I wish.

If I am inviting for a campus interview for the post doc position, is it acceptable to inquire about the salary again. Can the salary of a post doc be negotiable?
It is certainly fine to ask for clarification of what you were told about the salary and other financial details.  Keep in mind that nothing is certain until you have an offer in writing.

See How to handle salary negotiations while negotiating a postdoc position? regarding negotiation of postdoc salaries.  My guess is that the salary would be negotiable only within a narrow band, if at all.  If you get offered the tenure-track job, you shouldn't expect the postdoc employer to try to match it.

The main reason to choose the postdoc over the tenure-track job is if you think that the additional years of full-time research will help you get a better job afterwards, or improve your research success over the long run, and if you think this will compensate you for a couple years of a lower salary.  It very well might.

Keep in mind that a position that "focuses on teaching" will likely have teaching loads that leave you very little time or energy for research, and may also not give you much funding for that research.  I think people who take teaching-oriented positions very often overestimate the amount of research they will actually be able to get done.