(Just finished Mes. Archin - this is apropos of the siym). The Mishna in Archin (31) tells us that on the last possible to redeem a home in a walled city (the Torah allows the seller of such a home one year to repurchase his home from the buyer after which that right is forfeit) the buyer would hide to avoid giving the seller the opportunity to present him with the repurchase monies. Hillel therefore made a takkanah that the money be deposited in the Mikdash, and the seller retake his home even by force. The Ktzos (it seems I post a Ktzos almost weekly - maybe a once a week Ktzos blog is not a bad idea? Comments?) quotes an interersting chiddush of the Agudah based on this halacha. If one made an agreement subject to a condition being fulfilled within a time period, and one the last day of the time period some extraordinary circumstance (an ones) prevented fulfillment, the Agudah writes that one has no claim. After all, we see that if not for Hillel's takkanah, the fact that the buyer made himself unavailable on the last day would serve as no excuse to the seller - why did he wait until the last day when he had 364 days before that to take advanatage of the buback right?
The Ktzos (siman 65) disagrees with the Agudah. The Ktzos argues that in the case of arei chomah, a home in a walled city, the deal is finished at the moment of sale and the Torah simply allows the seller a right of un-doing the sale for 12 months. A claim of ones (extenuating circumstance) can serve to undo an action, but cannot make an action occur. To undo the completed sale, the seller has to see to it that the money is delivered - claiming ones cannot make an event happen. However, if there was a case where a deal was consumated on the condition that a certain event NOT occur over the next 30 days, e.g. if a seller sells a home on the condition that he does not return in 30 days, and due to extenuating circumstances, ones, he is prevented on the final day from returning home therefore fulfilling the condition and triggering the sale, the seller would have the right to claim ones and say that the fulfillment of the condition dues to duress is not valid and does not confirm the sale.
This Ktzos is discussed my many later achronim, so more maybe another time. Just one final note: if one understands the the idea of arei chomah is that the sale is not finalized until the final day of the year has elapsed and the seller has NOT offered to return the money to the buyer, then based on the Ktzos's own analysis, if the seller fails to turn over the money even on the final day due to ones, duress or extenuating circumstances, the sale owuld be cancelled.