That fee should reflect the present and the future value of the copyright. If you signed a work for hire and later want the copyright to the work, the only way you can get it is to negotiate with the copyright owner to transfer it to you. Finally, a work for hire will apply to all photographs taken on the assignment, not just to those used by the client. A transfer of copyright can be customized and apply to all the photographs or some portion thereof, such as only those used by the client. The copyright law allows someone to copy your work without penalty in certain cases.
Fair use is always subject to interpretation.
Each case has specific facts that must be examined before such a determination can be made. This is one reason why it is important to consult with a knowledgeable copyright attorney before jumping to conclusions about infringement. In recent years the trend has been to invoice the client with terms stating that the grant of rights to use the photograph is not in force until the invoice is paid in full. It should be understood that under this provision nonpayment may be both a breach of the client's contractual obligation and infringement of the copyright. This can create a legal question about the best way to enforce your rights - a question best answered by competent legal counsel.