Most fitting software license for customized client work?

Question

I am struggling with this question about a GitHub project of mine (MIT license). A client wanted me to customize it in a certain way and sell him the customizations (the customizations do not work without the base code though).

He will not be disclosing the source code, but he doesn't want to include any attribution or license information either, while still being allowed to legally use, sell and modify. Of course, I, on the other hand, don't want to take responsibility/give any guarantees/be liable.

I did some research and came up with the Unlicense

• http://unlicense.org/
• https://choosealicense.com/licenses/#unlicense

4 questions in direct connection with this issue:

1. As far as I understand it, he would be allowed to distribute the source code under this license as well, is that correct, and if so, can I limit that?
2. Is the Unlicense a good fit for the described purpose?
3. Are there any fitting alternative licenses/agreements?
4. Is me emailing him the code he bought together with the chosen License as textfile enough to complete the deal and make it legally bulletproof beyond a reasonable doubt?

Answer 1

It is not common to use open-source licenses for consulting work. Instead, the contract would contain a license from you to the client which covers whether and under which conditions the work may be used, modified, distributed, and sublicensed. The client may also want to own the copyright of the changes they paid for, in which case the contract would include a copyright transfer agreement. In any case, the contract will also cover topics such as liability. All of these details need to be negotiated with your client prior to the work. But this is about consulting, not about open source. Under all circumstances, ask a lawyer to draft a contract for you. That is the cheapest option in the long-term.

If you are the sole copyright owner of the open-source project, you can do with the code as you wish, including granting separate licenses to clients. The existing MIT license to the public is not relevant here, because it is not an exclusive license.

If you have accepted contributions from other persons, you do not own the complete copyright of the project and have to comply with the project license, even for your own use. Since the MIT license requires that you keep the copyright statement and license intact for each file, you may not remove this attribution. This doesn't prevent you or your client from selling or modifying the code, it just means that the source code must still include the notice.

You cannot relicense MIT-licensed code under the the Unlicense, since you may only use MIT-licensed works if you keep the notice intact. The Unlicense does not ensure this.