From a reading of the contract, it is clear that December 1 was not the time the sale would become final, but rather the time it could have been final had the money been ready. Under such circumstances, had there not been a clause about renting before the sale was complete, the buyer could not have demanded the keys before the actual closing, even though a large part of the money had been paid. Regarding the rent, there is some logic for the buyer to pay in a prorated manner, according to the percentage of the money that was not yet paid. (We will see below a halachic issue this can raise). In any event, since the contract refers to full rent, it is clear that until the apartment belongs to the buyers, they are to pay rent in full. (Since we had access only to the contract and your understanding of the events but not the buyer’s version, we cannot say anything conclusive or binding.)
The above is a brief analysis of your rights from the perspective of Choshen Mishpat.[3] However, you express uneasiness with the situation, in which the buyer paid a lot of money toward the apartment and yet still has to pay full rent until the time the sale was complete, even after the expected date of sale. This is a noble attitude, but in this case it requires us to delve into Yoreh Deah.[4]
Making down payments on merchandise that is not yet available but is to be legally transferred to the buyer later has the halachic status of a loan that the seller demands of the buyer prior to the sale. Therefore, the buyer/lender must not receive an actual monetary reward for advancing the money, as that reward would be ribbit.[5] If, for example, a buyer and seller agree that the price of the merchandise is $100,000 but that if the buyer advances significant funds, the price will be $98,000, then the $2,000 discount is considered a $2,000 interest payment.
A major exception to the prohibition on discounts for prepayment is when the commodity that is being sold is legally ready to be transferred, even if in practice the parties decide to delay the actual transfer.[6] To a certain extent, we consider that the sale went through and the payment is not ahead of its normal time.[7] Thus, you could have initially given your buyer a good price for a larger than normal down payment. You certainly could have offered him a reduction in the rent, either as a form of a permitted discount or following the logic that he bought a percentage of the apartment with his payments. We rule, however, that even when dealing with an existing commodity, one may not state two sales prices, where one is for a later payment and one for an early payment,[8] unless a heter iska[9] is used. Rather, the basic schedule of payments should be set first and then an exact sales price with that schedule in mind.
The contract that you provided does not seem to indicate anything but full rent. If there were a doubt about what was agreed or understood regarding the rental payment, a discount could be considered a compromise on the legal question and would not be ribbit.[10] However, it seems that there is no such doubt here and that all agree the contract indicates that you are correct; it is only that the buyer is now complaining and you feel bad that you held so much of his money for what ended up being longer than the expected time. If so, making it up to him by changing the agreement after the fact and forgiving the rent after December 1 would be a violation of the Rabbinical prohibition known as ribbit meucheret (late ribbit). (It is not a Torah violation, as this pertains only when the ribbit stipulation was set at the time of the “loan.”[11] The main practical difference between the two levels of ribbit is that if the interest payment was already made in an instance of Rabbinical late ribbit, the lender does not have to return the interest to the borrower.[12])