Can I ask for payment if my employer wants to use my dissertation?
I think your question is best rephrased as follows:


  If someone wants to use my dissertation, would it be ethical for me to ask for a payment?


Assuming you are asking about using the intellectual content of your dissertation, the answer is NO.  Whether or not that someone is your employer is irrelevant.

However, your actual presentation is protected by copyright, which for dissertations is normally completely held by the author.  So if someone wanted to use your specific words and/or figures beyond the limits of fair use, then you have the right to seek compensation.