According to this story, the city resisted vigorously until one day, to the great surprise of the Muslims, the gates were flung open and the city was opened up. The Muslims asked the defenders what had come over them, to which they replied, "You have shot us an arrow with a message that safety would be granted to us. We have accepted this and set aside the tribute payments." The Muslims replied that they had done no such thing, but after extensive enquiries they found a slave, originally from Junday-shapur, who admitted that he had indeed written such a message. The Muslim commanders explained that this was the work of a slave with no authority to make such an offer, to which the inhabitants replied that they had no means of knowing that and finished by saying that they were going to keep their side of the bargain, even if the Muslims chose to act treacherously. The Muslims referred the matter to [the Caliph] Umar, who responded that the promise was in fact binding, for "God holds the keeping of promises in the highest esteem." The moral is clear: even the promise of a slave must be respected.
Photo credit: Wikipedia/Zereshk - The interior of Masjid Jameh (Congregational Mosque) in Dezful, Iran.