Is it permissible for me to take from the money of my grandfather—the owner of the shop where I work for a meager wage—without his knowledge, an amount that meets my needs and suffices me, knowing that he possesses great wealth and the shop achieves immense profits?

It is not permissible for an employee to take money above their agreed-upon wage without the employer's consent; otherwise, they are considered a thief. If his grandfather forces him to work, he has the right to refuse if he is harmed or does not agree to the wage. As for taking his grandfather's money under the pretext of being forced, this is an invalid pretext.