Turning down a postdoc offer from a committee member. Can that create problem in the future? Recently I responded to a postdoc 'advertisement' posted by one of my PhD defense committee members. I still have 2 months to go for my PhD defense. However, I got an offer for a position from the said committee member. 

Now, I am in a fix about what would happen if I reject the offer? I am hoping to get a better position in a different university and group soon and I don't want to create any issues with my thesis evaluation due to the rejected offer.

Is it going to create an issue in the future? Can it influence my thesis evaluation?
A polite rejection to this post doc offer should neither hurt your thesis evaluation nor cause damage to your relationship with your committee member. The rejection email need not (and should not) be long or detailed, but as long as you make clear that you are honoured by the offer, that you have considered it carefully and seriously, but that you feel it is not quite the right path for you at this juncture, they will understand. I would suggest ending with a line saying that you look forward to finding ways to collaborate in the future.

All told, I think this is a positive sign for how this person will evaluate your thesis. They clearly think you are a worthy researcher or else they would not wish to hire you.