What to expect from or discuss in a Skype call about the possibility of a research position? I'm going to have a skype call with a University Professor (Computer Science) to discuss the possibilities of working in his lab as a postdoc.

He already told me (via email) that he can not produce an open position easily. So, he wants to know me better and to discuss the possibilities and research perspectives. Honestly, I do not understand what he means by his first sentence about the difficulty of "producing an open position". Does he mean "the chances are low, but let's have a talk"?

Also, I like to discuss and find common grounds for doing prospective research. But what would be a happy ending for such a discussion? For instance, he/I suggesting to work on a proposal? or finding a research project/topic to discuss later in more detail? To be honest, I'm not sure how to lead such an (somehow) informal meeting to a more serious opportunity.
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.