I wrote some texts and put then in the Wikisource website, which says that the content is available under both CC-BY-SA 3.0 and GFDL.
Suppose a certain publisher wants to create a printed book based on my texts. Creating the book will probably require a lot of investment for copyediting, design, marketing, etc. So to cover his investment, the publisher will probably want to have exclusive rights to publish the contents for some time.
I understand that the SA clause prevents him to do this, since this work is a derivative of my work, so he must publish it under the same license.
But what if I, as the author, allow him to ignore the SA clause? I want to do this since I want the text to be published and I understand that most people will not want to invest unless they can get some compensation.
EDIT: to clarify: my question is specifically about derivative works. Suppose person X copies texts from wikisource, does some editing, and publishes the edited material in a book. Now person X wants to prohibit person Y from copying the edited material from the book (not the original material from wikisource). The SA clause prohibits him from doing so, since he is required to publish the derivative work under the same license. My question is: can I, as the author, allow person X to ignore the SA clause and prohibit person Y from copying from the book?