Does this belong to copyright infringement?

[Milady]

Hi,
My friend has a page on facebook, & he asks me to contribute around 20 first pages of a (comic) book that I have. It's about 10% of the content. If I post the said pages on his page, will I get any copyright-related problem? He told me that, since it's his page, & I don't use my complete real name on my fb anyway, there should be no problem at all. He also says that it's a japanese comic & I'm in Spain so I really has nothing to worry about.

Is it then okay to do that? I don't want to be stingy but I don't really know about copyright, so thanks for any help.
P.S. His facebook page is about Japanese manga, and there are about 200 people (mostly his friends) who "like" the page & able to discuss and see the content.
P.S.2 I don't want to post stuff ilegally, but in fact, it's to promote the comic concerned. That's why instead of the whole book, my friend & I only want to post the first 20 pages, like a teaser. But if it's not okay then I won't do it.