Bava Kamma chapter 9, Mishnah 3. In this Mishnah we deal with the responsibility of a person hired to do a job - a craftsman or artisan - for the item handed over to him.
Two levels of responsibility are placed upon the craftsman:
Shomer sachar - A paid bailee: The one who hires a craftsman for a job deposits his item in his hands, and the craftsman is considered as someone who is there to watch the item and receives payment for this. Therefore, he is liable for theft and loss, if the item is stolen or lost.
Adam hamazik - A person who causes damage: Furthermore, if during his work on what was given to him he caused damage, he must pay based on the regular and familiar law of a person who causes damage, who causes damage directly and is responsible for his actions.
"Natan la'umanin letaken vekilkelu" - He gave to craftsmen to repair and they ruined it:
The question that arises in our Mishnah is how we calculate the payment. The homeowner gives the carpenter wood worth ten dollars and asks him to build a table. The carpenter sets a price of fifty dollars for his work and builds the table. A moment before handing it over to the homeowner, during the final polishing, he drives in an extra nail - and the whole structure collapses and falls apart back into a pile of boards.
Does the carpenter pay for a broken table worth a hundred dollars, or does he actually pay nothing, since he received wood in his hands and everything returned to a pile of wood?
The Mishnah's answer is that he must pay for the damage he caused, since he is a person who causes damage: with his hammer he broke the table. The meaning of this is that the Mishnah holds "Ein uman koneh beshevach keli" - a craftsman does not acquire ownership through the improvement of a vessel. The improvement made to the raw materials immediately becomes the property of the employer who hired the worker, and is not the property of the worker.
The payment calculation:
Upon finishing the construction of the table its value was a hundred dollars, and its owner is the employer.
With the final blow the carpenter broke a hundred-dollar table, and its value dropped back to ten - a damage of ninety.
On the other hand, his work was already done, and the employer owes him fifty dollars in wages, and this amount is deducted from the damage.
The bottom line: the carpenter pays the employer forty dollars for the damage.
In practical halachah, the halachic authorities debated this question extensively: is the craftsman the owner of the financial value of the improvement prior to handing over the product, or is he not its owner.
Handing over a finished product - "Shidah, teivah, umigdal" - a chest, a box, and a cabinet:
The Mishnah continues and discusses a case in which the homeowner does not hand over raw materials but rather a finished product:
"Shidah" - a sort of chest in which a person is carried around the city, though in other contexts it means a storage chest.
"Teivah" - a storage box.
"Migdal" - a cabinet.
In all these cases, we are dealing with a finished piece of carpentry given to a carpenter for repair, and instead of repairing it, he damaged it. He certainly must pay for the damage, and it is a simple logical deduction: if he is liable for damage he caused prior to delivery, he is certainly liable for damage he caused to a finished product.
The contractor who demolishes a wall:
The third case is slightly different. A builder takes on a job as a contractor - meaning he does not receive an hourly wage like a standard worker, but a set amount for the job - and his task is to demolish a wall. The assumption is that the demolition will be done in such a way that the bricks or stones from which the wall is built will remain fit for reuse. However, he did not demolish it properly and broke the stones, rendering them no longer usable, or partially damaging them.
The Mishnah adds a case where the stones fell and damaged a third party, a passerby walking in the street. Whether he damaged the homeowner's property or injured the pedestrians passing by there - the contractor who took on the project bears the responsibility and must pay. Had he been considered merely a worker, the ruling might have been different, as he would not have taken on the responsibility of protecting passersby; but as a contractor, his responsibility is absolute.
He demolished half a wall and the other half fell:
"Patur" - if he was asked to demolish half of the wall and the second half fell on its own, it is not his fault: he did as instructed, and he had no reason to assume that the second half would fall. This is considered an unavoidable accident - he acted responsibly, and accidents happen; what occurred was beyond his control.
"Chayav" - if the second half fell because he struck it with excessive force and knocked it down, he is liable, for he is a person who causes damage, and a person is always considered fully responsible for his actions.
It is worth emphasizing: not only is the contractor liable when he was negligent and careless, such as breaking the wall improperly, but even if he did nothing - if the shidah, teivah, and migdal or any other object were stolen or lost - the craftsman is liable to pay, because he has the status of a paid bailee, and he is obligated to guard the property entrusted to him by the homeowner and not let it be lost.
In conclusion: In this Mishnah we learned that the craftsman bears two levels of responsibility - as a paid bailee who is liable for theft and loss, and as a person who causes damage who is liable for damage he caused during his work. We discussed the principle that "a craftsman does not acquire a share in the item's improvement", which implies that one who ruins the vessel before its delivery pays the full difference minus his wage; we saw the logical deduction regarding damage to a finished product, and the full responsibility of the contractor who demolishes a wall - both toward the homeowner and toward pedestrians - except in a case of an unavoidable accident, where he is exempt.