John Calvin on Numbers 35:28-29

28. Because he should have remained in the city of his refuge. The period of banishment is prescribed, “until the death of the high-priest,” because it would have been anything but humane that all hopes of restoration should have been cut off from the unhappy exile; and, when a new priest succeeded to reconcile the people to God, this renewal of grace was to propitiate all offenses. Wherefore it was not unreasonable that God should entirely restore those who were only punished for inadvertency.