Turning down a postdoc offer from a committee member. Can that create problem in the future?
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.