Is the commission workers receive from the tractor driver for delivering bricks after they have been placed, with the factory's knowledge, permissible or forbidden? And if delivery is part of their core duties, is taking commission forbidden? And if the driver gives it of his own accord, is there any impermissibility in that?

It is impermissible to take a commission from the tractor owner unless the employer gives permission. If the plowing of the land is part of the official work, then taking another wage for it is an injustice and consumption of wealth by falsehood. And if it is not part of the official work, then it is impermissible to do it during official working hours, because the employee's time is owned by their employer.