My potential future PhD supervisor ask me to peer review a paper I have been working for two years after my Master and I am now looking for a PhD. I got a PhD offer from one university, met the person who would supervise me and told them I am very interested, while making it clear that I also applied to other PhD offers and am waiting for answers. They said that that it was not an issue for them.

But now he just sent me an anonymous paper and suggested to me peer-reviewing it to help me "get a foot on the ladder". I am really confused by this proposition since: 


I am still working for my company
I am still searching for PhD offers to apply, which takes me all my free time, 
I never reviewed a paper before. 


So, I am not really in the mood to do this now since I have more urgent concerns. However, I do not want to upset this potential supervisor, nor make them believe that I am not motivated.

So, should I do this review or not, and if I don't, how can I refuse without upsetting this potential supervisor?

Let me precise that I am likely to do my PhD there if my other applications don't get accepted.

EDIT

The paper in question is anonymous and I am asked to peer review it for a journal publication. The paper subject is in my field of research but not really in the field of my potential supervisor.
Maybe it's a matter of question formulation, but I find this to be a very peculiar request and would be equally confused as you seem to be even if I wasn't busy with other stuff. 

Specifically, you're not qualified to review a paper, yet.

If he's asking your analytical opinion on a paper, kind of like a journal club style dissection of the content, then sure I guess that's no big deal. But if you are given a manuscript, written by someone else and submitted to a journal, I find this request unrealistic and even unethical. 

Also, in a way disagreeing with the previous answers here, I don't think it's a reasonable request even if you were given the article in a more relaxed, journal club-like format. Because you are supposed to learn to read and understand the scientific literature, it's part of your training as a PhD student. To use that during the recruitment process, as a test is a bit unfair in my eyes. 


  that was my first feeling actually, that he was assigning me some of
  his work while I am still in the application phase. The paper is
  anonymous and I am asked to peer review it for a journal publication.


The above alone, would even deter me from considering to start there. I think it might be worth thinking about what kind of work environment you want to get yourself into for the coming 3-X years of your life.

But again, it's possible that I misunderstand something in the OP. Just my two cents...