What should I do if the first author wants me to write the whole paper? I have finished three years of work in a Ph.D. program. I've published one conference paper as sole author, and have recently sent out another paper to a prestigious conference. 

I started a project last October with a professor who does not have anything to do with my dissertation. He asked me early on to do a lot of work so I asked him who is the first author, and he said he would be. I've already written the full literature review for this journal paper, and came up with the framework by myself. He has only advised me on it.

Now, I have a clear idea about my dissertation, and I have to write my proposal and defend it this fall. So, the work with this professor is rather burdensome to me right now. So far, he has not worked on it at all. I've written six or seven out of the 15 pages of the journal paper. The main thing left is data analysis. He wants me to do the data analysis and write it up as well, and recently he also asked me to write introduction of the paper. At this point, I thought, he wants me to write the whole paper, which he can edit at the last moment and become first author.

Since this paper does not have anything to do with my dissertation, I do not want to work with this paper anymore. I told him I am busy with my own research, it seems like that he will wait until I finish writing the paper.

What should I do? I do not want to cut the relationship but he is asking too much which I think is unethical. Any strategic advice will be appreciated. 



UPDATE:
I talked to the professor and said there must have been misunderstanding about the responsibilities as a second author in my part and as a first author in his part. I said that I understood that writing literature review will meet the responsibility as a second author. 

His brief comment on the e-mail was that he wrote a conference paper already, and this journal paper (that I'm working on) is a developed version of the conference paper. So, he did the major work already for the conference paper that is why he is the first author. And he stated that I am supposed to write a literature review and to do the data analysis for this journal paper. 

OK. He has integrity in his position. I remember that he said something like this when we started this paper 10 months ago. But the problem was that his position was not re-enforced again, and the lit review I've done took off to a different terain. That means, I did a lot of creative interpretation of the framework. (Sorry, to technical folks. This is social science study using business management theories. The upfront literature review is very significant. My dissertation uses machine learning and NLP stuff. yes, this paper is unrelated with my dissertation). 

Anyway, the reason I'm writing this update is to provide the professor's perspective that I've got to learn recently. So the major lesson I've learn through this experience is to keep reinforcing (in emails) about my understanding of my and his responsibilities on this project.
First of all: Good call to talk about authorship issues at the beginning, rather than only once troubles started! Now authorship issues do occasionally have to be re-negotiated depending on the course the project takes.

There do seem to be potentially two separate aspects in your question:

A. What to do if I have contributed as much as I am willing as a secondary author, yet the first author keeps asking for more?
- Point out that you have done X, Y and Z, and that you are only willing to do significantly more if you are the first author (fundamental reading + commenting remain unaffected of course).

B. I started a side-project, and I no longer want to work on this AT ALL.
- Make clear to your co-author that you have no intention to work on this project anymore, and that this pertains to the future, too. Either give them free rein to continue the project without you (either with you on the author list or not, depending on your preferences); OR make sure you are still available for basic reading/commenting, but simply nothing more extensive.

In both cases, if the coauthor shows a significantly stronger reaction than "medium annoyance", then maintaining a positive relationship to them is futile anyway.