I agree that your professor's act is not ethical (if his purpose is for you to ghostwrite the review) and you should politely decline to review it. However, I have seen this happens a lot. Not just with peer-review papers, but with MS and PhD dissertations, book chapters, proposal etc.
If it was to up to me, I would do the following. Read the paper, try to do a short research and see if you can understand its concept and/or purpose. Since it is outside your field, most likely it will be very challenging and you won't get it, but that is totally fine. Remember, maybe your adviser wants to see how you would handle such task, or if you willing to dig outside your comfort zone and come up with solutions (in this case, a review!). Remember, some professors like that.
Also, I would like to point out, that even if you ghostwrite a review to your adviser, that does not mean that he is going to submit the EXACT document you prepared. I have witnessed many cases in which the professor uses such document as a base or blue-print and then add/edit as necessary after s/he reviews the paper. This is exactly what my advisers used to do. They would give me a paper, I will review it then they would discuss my review (while I'm in the office with them) to teach me how to and/or not to think, review, write and analyze.
Finally, you might find this tip useful. If you couldn't review the paper because it is very hard, outside your field, too complicated or any other reason. All you can do is to go over the language, editorials, referencing, citations, formatting and base you review around such observations. Maybe doing this would send a message to your adviser that you could not review paper because (as you might have told him), it was outside your field. At least doing so can be interpreted in way that shows that you tried and did your best (although it can also be interpreted in a way that you were lazy, [let's hope not!!]).