The tāʾ has been assimilated into the ẓāʾ. ʿĀṣim, Ḥamzah, and al-Kisāʾī recited it with an ungeminated ẓāʾ: they omitted the tāʾ of the reciprocal form while retaining the tāʾ of address, as in His saying, Exalted is He, “Do not cooperate.” Both readings have the same meaning: you support one another. A ẓahīr is a helper.
“...in sin and aggression”: that is, disobedience and wrongdoing. “And if they come to you as captives”: Ḥamzah recited, “asrā”; both forms are plurals of “captive” and have the same meaning. “You ransom them”: that is, with money, thereby securing their release. The people of Medina, ʿĀṣim, al-Kisāʾī, and Yaʿqūb recited it in the sense of, “you exchange them”—that is, exchanging one captive for another. It is also said that the two readings have one and the same meaning.
Regarding the meaning of the verse, al-Suddī said: Allah, Exalted is He, took a covenant from the Children of Israel in the Torah that they should neither kill one another nor expel one another from their homes; and that whenever they found a male or female slave from among the Children of Israel, they were to purchase that person for whatever price was required and set him or her free. Banu Qurayẓah were allies of al-Aws, while Banu al-Naḍīr were allies of al-Khazraj. They fought one another in the war of Buʿāth: Banu Qurayẓah and their allies fought Banu al-Naḍīr and their allies. Whenever they prevailed, they destroyed their homes and expelled them from them. Yet, whenever a man from either group was taken captive, they would collect money for him until they ransomed him, even if the captive was from their enemies.
The Arabs would reproach them, saying, “How can you fight them and then ransom them?” They replied, “We have been commanded to ransom them.” They would say, “Then why do you fight them?” They replied, “We are ashamed for our allies to be humiliated.” Thus Allah, Exalted is He, reproached them for this, saying: “Yet here you are, killing one another.”
There is transposition in the verse, whose order is: “You expel a group among yourselves [15/a] from their homes, supporting one another against them in sin and aggression, although their expulsion is forbidden to you; and if they come to you as captives, you ransom them.” It is as though Allah, Exalted is He, had taken from them four covenants: to refrain from fighting, to refrain from expelling one another, to refrain from aiding their enemies against them, and to ransom their captives. Yet they disregarded them all except the ransoming.
Allah, Exalted is He, said: “Do you then believe in part of the Book and disbelieve in part?” Mujāhid said: He means that if you find him in another’s possession, you ransom him, while you yourself kill him. “What, then, is the recompense of those among you who do this”—O assembly of Jews—“except disgrace”: that is, punishment and humiliation, “in the life of this world.” Thus, the disgrace of Qurayẓah was slaughter and captivity, while the disgrace of al-Naḍīr was evacuation and exile from their homes to Adhriʿāt and Arīḥā in Syria. “And on the Day of Resurrection, they will be returned to the most severe punishment”: the punishment of the Fire. “And Allah is not unaware...”