I mean a basic script like a 2d movement, I have one already developed and It would be tedious and difficult to create a new one without taking the same path.

honestly it's okay, like if you did the original code yourself and just copy/paste it in your new project it's perfectly fine for me. If you want to be "fair" (but if you don't it's okay), just add 5-10mn to your total dev time at the end or precise it in the description of your game ;)