What is the ruling on workers in a building materials factory taking a financial bonus from a contractor for dividing a quantity of building materials into separate piles instead of placing them in a single pile?

The bonus for an employee takes the ruling of gifts to employees; therefore, it is not permissible except with the employer's permission, due to the prohibition of the Prophet (peace and blessings be upon him) regarding that. The employee is a private hire (ajīr khāṣ), and it is not permissible for him to work for anyone other than the employer during working hours except with the employer's consent, because the time of a private hire during working hours belongs to his hirer. Exceptions to this include what the Sharia has excepted, such as prayer, or what custom has excepted, such as meal times, or what has been excepted by a condition. So, if the factory owner permits taking the bonus, then there is no objection; otherwise, it is not permissible.