The document you'll need is an assignment of copyright, which is a permanent transfer of rights. It must be in writing (not an oral agreement).
Basic assignment. Below, we provide a very basic copyright assignment that should work for you. (For more sophisticated assignments, see our Getting Permission book.) Your co-author is the "Assignor" and you are the "Assignee." Only the Assignor needs to sign the assignment agreement. If you are paying for these rights, you should acknowledge your payments to your co-author somewhere in the agreement.
What if you signed a co-writer agreement? If you have a written co-authorship agreement (sometimes known as a co-writer or co-ownership agreement), you may have to follow the requirements of that agreement as to resolving disputes or other co-ownership issues.
Assignment of Copyright By Co-Owner
I, _________________________ (“Assignor”), am co-owner of the work entitled ______________ (the “Work”) and described as follows: _______ _____________________________________. For consideration which I acknowledge, I assign to ______________________ (“Assignee”) and Assignee’s heirs and assigns all of my right, title, and interest in the copyright to the Work and all renewals and extensions of the copyright that may be secured under the laws of the United States of America and any other countries, as such may now or later be in effect. I agree to cooperate with Assignee and to execute and deliver all papers as may be necessary to vest all rights to the Work.
Signature of Assignor