Question
I hired a contractor to renovate my apartment. The contractor agreed to pay for any damages that his workers do, as if he did the damage. I showed the contractor that I stored my Succo boards under one of my porches. Due to their carelessness, the contractor's workers sealed the Succo boards with cement. It would cost me more money to undo their work than the value of the Succo boards. I should mention that I still owe money to the contractor for his other work so if there is a doubt perhaps, I can withhold payment.
Answer
Since the way that the contractor's workers damaged you is by rendering your boards inaccessible, we will begin by examining whether one is liable for damages that he caused by rendering someone's possessions inaccessible.
The case in which the Gemara (BK 98A) discusses this issue is where A held a coin in his hand and B hit his hand from the underside causing the coin to fall from A's hand into a body of water. The Gemara cites the Amoro, Rabbo, who rules that B is not liable for damages since he can argue that nothing happened to A's coin, saying to A: "It is right in front of you."
The Gemara qualifies that B's argument is only valid if the water is clear since then the coin is visible. But if the water is sullied and the coin is invisible, B's argument is not valid and he is liable for the loss of the coin. Tosafos (beg. wo. Akurin) explains that the critical fact is not whether the water is clear or cloudy but accessibility. If a diver can retrieve the coin, then B is not liable. But if it is not retrievable then B is liable. The reason the Gemara spoke about whether the water is clear or not is because generally when the water is clear the coin is retrievable and if not, the coin is not retrievable.
In your case the boards are retrievable but it is not financially worthwhile to retrieve the boards. We have to investigate whether to classify your boards as being accessible or inaccessible. The Imrei Binah (Dayanim 39 be. wo. Ach) explicitly writes that even if it costs more to retrieve the coin than the coin is worth, the coin is considered accessible. He explains that the critical issue is whether the coin left the possession of its owner. If it left his possession the one who damaged cannot tell him that his object is "right in front of you." Only if it is absolutely irretrievable is the coin considered as having left its owner's possession. However, if it is in some way accessible, it is still in his possession even though it is not financially worthwhile to retrieve the coin. Therefore, according to Rabbo the contractor is not obligated to pay for your loss.
We note that the Ketsos (386, 9) says that there are Rishonim who maintain that the criterion is really what it says in the Gemara, namely whether the coin is still visible or not. Even if it is inaccessible, if it is visible B is not liable. If the Ketsos is correct, (Several Acharonim disagree with him.) according to these Rishonim, the contractor is certainly not obligated to pay you if your boards are visible.
The Rishonim dispute whether the position of Rabbo is authoritative. The ruling of the Rif and Rambam (Choveil 7, 11) and consequently the Mechabeir in SA (386, 1), is that the consensus is not like Rabbo. B is liable for A's loss since this type of damage is garmi and the reason Rabbo ruled that B is not liable is because he maintains that one is not liable for garmi.
However, the position of Tosafos (BB 22B) and other Ashkenazic poskim including the Ramo (386, 3) is to rule like Rabbo. They maintain that Rabbo agrees that one is liable for garmi but Rabbo holds that these damages are gromo and not garmi.
Thus, we have established that according to the Ramo the contractor is not liable.
We should note further that there is a difference between your case and the case where B struck A's hand causing the coin to fall. In that case B acted on the "damaged" object since he effectively threw the coin into the water albeit in an indirect manner. However, in your case the workers did not act on your boards at all but merely rendered them inaccessible by surrounding them with cement.
The Rosh (BK 9, 13) writes that one of the conditions which must be fulfilled in order to classify a causative act as garmi is that the one who caused the damage must act on the object that was damaged. He proves this condition from the ruling of the Gemara (BK 26B) that if A threw a breakable object down from a roof onto cushions and B removed the cushions, B is not liable since his act only caused the object to break. The Rosh explains that the reason B's action is not classified as garmi is because B did not act on the object that broke.
Similarly, in your case the workers did not do anything to your boards. This a second reason why their action is classified as gromo and not garmi.
A third reason that the workers only damaged by gromo is because their action is only preventing you from using your boards. The source that this is gromo is another ruling of the Rosh (BK 2, 6) that is ruled by SA (363, 6). If A evicted B, the owner of a house, from his house, and then prevented him from living there, A is not liable even if as a result B had to rent another house, since A only caused B an expense in a manner of gromo. The reason is because the Rosh, whose position is authoritative, maintains that preventing a person from using his property is classified as gromo.
Thus, we have three reasons to classify the workers' action as gromo. Therefore, beis din would rule that the workers, and consequently the contractor, are not liable.
However, the Gemara (BK 55B-56A) lists various actions for which beis din will not make someone pay but nevertheless he is liable in the heavenly tribunal-chayav bedinei shomayim. Rashi explains that the reason beis din does not make the person pay is because his action is gromo. Thus, even though beis din cannot force your contractor to pay you, nevertheless he is chayav bedinei shomayim like many other cases of gromo.
There is a dispute whether when one is chayav bedinei shomayim the victim can by himself take money from the perpetrator to cover his loss. The Maharshal (Yam Shel Shlomo BK 6, 6), who is cited and agreed to by the Shach (28, 2), maintains that the victim may not take money. However, Rabbi Akiva Eiger (glosses thereon) cites Rishonim like the Rashbo and Ran who maintain that he may take money, and Rabbi Akiva Eiger remains with a doubt whether the victim can say kim li like this opinion.
We note that besides the Rishonim who are cited by Rabbi Akiva Eiger there are others like the Maharach Ohr Zorua (res 229) in the name of the Sefer Hamekutso'os (Rabbeinu Chananeil) and the Maharam Mintz (101) who maintain that the victim may take money.
Furthermore, whereas Rabbi Akiva Eiger remained with a doubt whether one can grab and claim kim li, the Maharsham (1, 160) and the Erech Shai (end 28, 2), based on these poskim, rule that beis din cannot force one who took money to surrender what he took since he can claim kim li.
You are in a better position since you do not need to take money and can merely withhold payment (tefeeso beheteir).
In conclusion: Even though there are three reasons why the damages that the workers caused are only gromo and not garmi, nevertheless you may deduct the current value of the (used) boards from the money you owe the contractor.
