I'm currently working on a wordpress plugin. My client gives me a specs doc (a powerpoint presentation, if you can call that a specs doc), and I code the requested functionality.

Every time I send him code every file containing code starts with these lines:

Author: My Name
Copyright: The_client's_company.com
License: MIT Expat (http://en.wikipedia.org/wiki/Expat_License)

My intention being giving my client complete right to relicense and distribute the code under any other license (as the TOS of the freelancing website requires, plus I know he intends to sell it under a proprietary license), but at the same time giving myself the right to expand and redistribute the plugin under MIT license if I wish to (not that I do).

The reason is I am paid only 10USD/hour (this is my first gig) so I want to at least keep the right to reuse parts of the code in other projects or expand it if I want to start a similar project myself when I finish the contract (unlikely, but who knows...) or show it to potential employees. The contract we agreed upon doesn't include any licensing specifications but I've informed him on the emails we've interchanged that although all my work is licensed by default as MIT I'm giving my clients the copyright of the code I produce so they can relicense it at will before distribution.

Is this the correct way of achieving that?