Here's a different perspective. I'm going to assume that the course you're teaching is a graduate-level course from the description that you've given, but what I'm about to say applies even if it isn't.
I have actually seen this happen in top-tier American schools too. In this case, things were slightly different. Namely, a postdoc supervisor was listed formally as an instructor for a graduate-level course, but when you actually went to the class, someone else would be there to teach the course (and often the professor would not even be present).
In this particular case that I'm thinking about, it happened because of bureaucratic reasons (certain postdocs were not formally allowed to teach high-level courses), and although they were not being paid extra for it, the fact that they were allowed to list this on their CV as a "teaching experience" was enough for the postdocs to get into this arrangement, even without the extra pay.
So if you think about it, things are fairly similar between you and these postdocs. You are both teaching a course that is not credited to your name, and you are both not getting paid. Most likely you are not allowed to formally be an instructor for a course for graduate students if you are a graduate student yourself.
Given the fact that you did not claim in your original post that the advisor is trying to take all the credit for your work, maybe he is trying to give you some solid teaching experience (which is important if you are planning on looking for academic jobs). In fact, this experience (assuming that your advisor allows you to put this on your CV; I don't see any signs of why he wouldn't let you do this) is going to look fantastic in the future. It shows the tremendous amount of trust on your advisor's part, and he believes that despite being junior, you are fit to teach graduate-level courses.
So, it's good to stand up for yourself and make sure that you aren't being exploited, but unless you see true red flags, it is also good to trust your advisor before everything, because if there is one person in the academic world who wants you to succeed the most, it is your advisor.