Is it permissible for an employee to sell an invention to his company—through an intermediary—knowing that the company did not reward him for previous inventions, and the invention will save it money?

Firstly: The rights of invention, innovation, and authorship are private rights belonging to their owners, and they are recognized by Islamic law and have financial value in contemporary custom. It is not permissible to infringe upon them.

Secondly: If the invention was made independently of the company (outside of work hours or without relying on its resources), then it is your exclusive right, and you may sell it to whomever you wish. However, if it was made at the expense of your work or using the company's resources, then you have done wrong and must compensate the company. The principle is to be transparent with them and reach an agreement; otherwise, you must ensure their right reaches them by any means after its evaluation. It is not permissible to infringe upon the company's right under the pretext that they do not appreciate you.