Q. One Friday, I rented a car for a month. Immediately upon leaving the city for the Catskills, I noticed that I had a big problem: the air conditioner wasn’t working. I did not have enough time to return to the rental location to ask for a replacement before Shabbos, so I continued on to the mountains. When I return to the city on Monday morning, may I demand that the company cancel the rental or give me a different car?
A. When a person buys something, he may not arbitrarily decide to void the sale. Similarly, once someone makes a kinyan for a rental period, he may not void the rental.
Nevertheless, just as a buyer is entitled to nullify a sale if the item is defective enough that most people would consider it worth voiding the sale (Shulchan Aruch Choshen Mishpat 232:6), when someone rents an object that is similarly defective, the rental is void (see Nesivos 312:1).
There are certain defects, however, that would be considered reason to invalidate a sale but not a rental. In the case of a sale, if the buyer used the object after noticing the defect, he forfeits the right to void the sale, and he may no longer renege (Shulchan Aruch 232:3). The question is whether your usage of the car for two days similarly prevents you from voiding the rental contract.
There are several reasons why it would not prevent you from doing so. To understand them, we must first examine why someone who uses a defective item he bought forfeits the right to cancel the sale.
Some poskim explain that since a person who borrows something without permission is considered a thief (ibid. 292:1), if a person who bought an item that he plans to return uses it in the interim, it would be considered theft. We therefore assume that he decided not to void the sale so as not to be a thief (Galya Mesechta 10, cited in Pis’chei Teshuvah 232:1; Divrei Geonim 6:10, according to Nesivos 232:1).
This approach likely applies only to a sale, because the buyer has no right to use the object if he doesn’t accept the sale as final. In the case of a rental, however, even if the initial rental contract was for a month, if he uses it for two days it is not considered theft, because the item is meant to be rented out and it is a favor to the owner of the object if it is rented for just those two days because he probably wouldn’t have found someone else who would agree to rent it with that defect (Shu”t Teshuras Shai 285; see Pis’chei Choshen ch. 6 fn. 29 for another approach that differentiates between rentals and sales).
Other poskim write that the reason a buyer may not void the sale if he used the object after noticing the defect is that this usage is considered a mechilah (forgoing the right to a claim). This approach would apply to rentals just as much as it applies to sales (Ra’anach 39, cited in Shach 232:3). [It is possible, however, that a broken car air conditioner during the summer months is such a bad defect that your usage is not considered a mechilah; see Shulchan Aruch 155:36 for a similar scenario.]
In your case, though, there is a clearer reason why you may void the rental.
The halachah is that if someone buys a horse, and on his way home he notices that it is wounded, and he rode the horse back to return it, that usage is not considered a mechilah because he is considered an oness (compelled to act) because he had no other way to get it back there (Pis’chei Teshuvah 232:1).
Since you did not have time to return the rental before Shabbos, your driving it to the mountains and back is not considered a mechilah, and you may void the rental (Teshuras Shai loc. cit. and Shu”t Maharsham 4:108).
