Halakhah sur Les Nombres 35:32

He prohibited the taking of ransom from an unintentional killer, so as to exempt him from exile. Rather he must be exiled under any circumstances. And that is His saying, "And you shall not take ransom for him who fled to the city of his refuge" (Numbers 35:32). And the regulations of this commandment have been explained in Makkot. (See Parashat Masei; Mishneh Torah, Murderer and the Preservation of Life 1.)

Not to take ransom from one liable for exile to exempt him from exile: Not to take ransom from one liable for exile because he killed inadvertently, to exempt him form exile, as it is stated (Numbers 35:32), "And you shall not take ransom from the one who fled to his city of refuge to return to dwell in the land." And according to this literal meaning, it appears that 'the one who fled' is [in the past], meaning to say, do not take ransom, from one who has fled to his city of refuge, to return to dwell in the land of his ancestors' dwelling.