What to expect from or discuss in a Skype call about the possibility of a research position?
You need to answer two questions here:


What is it that you want out of this?
What can you offer this person?


In answer to question 1, if it's just that you want any post-doc, then this conversation will likely go poorly for you.  

The professor "cannot produce an open position", meaning that he is uninterested in working with just another researcher to get more people in his lab.  He has an obvious interest in working with someone, but he isn't sure if it's you.  He'll want to know that you two share the same research interests, that your skills are up to the task, and that you can contribute positively to the field and your mutual reputation as researchers.

In response to question 2: The prof most likely wants to know if you want this specific post-doc position because this professor is a leader in the field that you are in and you will learn a lot from this person, and also that you will be able to make a significant contribution without him holding your hand, but that gets him some recognition.  You need to make sure to communicate that.