Does volunteering for extra work in the same workplace compensate for shortcomings in performing original duties and make the salary permissible?

The hirer owns the usufruct of the special employee throughout the work period. The employee must perform what has been agreed upon, and any shortcoming in the entrusted work is not permissible শরعاً (religiously/legally). The hirer has the right to deduct from the employee's salary an amount commensurate with this negligence. The shortcoming in the agreed-upon work cannot be compensated by a different work unless there is an agreement and permission from the hirer.