Script to add to a web design to give access even if Cpanel password is changed

I developed a website for someone who is a middleman between I (the developer) and the owner. But he is presenting himself to the owner as the web developer/designer.

He has gotten the hosting and domain already and now he said the owner wants him to upload to the hosting he has already bought before he pays him. And it is from whatever is paid that he will also pay me.

My issue now is I don't know him before as this is our first project together. If I go ahead to give him the design, I am afraid I may start hearing another story. And he said the owner who is a novice does not want to check the design on any other sub-domain but his own.

Is there a script like a backdoor that I can secretly include in my design (before giving it to them) and later use the backdoor to get access back to my design and hack it or wipe it off without knowing their cpanel password, in-case they refuse to pay me later.

Please help on this.