[The ruling on cancelling (the claim for) one slain person in exchange for another slain person who is not the killer]

The answer – and Allah is the One who grants success – is that such a reconciliation happens very often among the tribes of Yemen, and they have become accustomed to it. If the tribe had killed, in its revenge, the actual killer, then the reconciliation would be Sharʿī, with no objection to it. If we want the reconciliation mentioned in the question to be Sharʿī, then we must oblige the first killer to hand over a full blood-money to the heirs of the slain man, and likewise oblige the second killer to hand over a blood-money to the heirs of the other slain man – unless the heirs from each of the two tribes are pleased to waive their right to the blood-money, or to waive part of it. In that case, the reconciliation is Sharʿī and there is no objection to it.
Furthermore, we say: the reconciliation mentioned in the question is good and permissible; indeed, it may even be obligatory insofar as it puts a stop to tribulation. The scholars have explained His saying, exalted is He: "And if two factions among the believers should fight, then make settlement between the two. But if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah. And if it returns, then make settlement between them in justice and act justly. Indeed, Allah loves those who act justly." [al-Ḥujurāt:9] They said: what is meant by the reconciliation first mentioned in the verse is the halting of the fighting, without looking (yet) at what each group is entitled to from the other of blood-monies, blood (claims), and wealth.
Then they said: what is meant by the reconciliation mentioned second in the verse is that each of the two groups be given in full what it is entitled to from the other of blood-monies and compensations for injuries against life and property.
Thus, the reconciliation mentioned in the question is an instance of the reconciliation first mentioned in the verse, while the second reconciliation remains.
It was incumbent upon those who reconciled (the two sides), after the first stage of reconciliation, to reconcile a second time, by examining what had been lost from both sides, so that everyone from whom something had been lost be given compensation for what he lost – whether what was lost was a life, or property, or a limb, or a wound.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2