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.