Bamidbar Perek 35:32 "And you shall not take a ransom for him that has fled to his city of refuge, that he should come again to dwell in the land, until the death of the priest."
Rashi
Monetary compensation (to the court or to the Go’el haDam) cannot exempt an unintentional murderer to be able to return to society.
Ramban
Even after one has already lived in the City of Refuge, one cannot pay compensation to leave the city prior to the death of the Cohen Gadol. One might think after living in Galus, circumstances would occur where the unintentional killer could return to society without worry; nevertheless, one must complete the term of Galus.
Sifri
An intentional murderer also could not provide monetary compensation to be exempt from the death penalty. The repetition of not taking compensation from the unintentional murderer to escape punishment of Galus signals that the intentional murderer cannot provide compensation after the fact to change the death penalty to one of Galus.
Saadiyah Gaon
Do not take monetary compensation from an intentional killer that kills with the intention to live in a city of refuge until the Cohen Gadol dies (in lieu of the death penalty).
Bamidbar 35:33 "And you shall not pollute the land wherein you are; for blood, it pollutes the land; and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it."
This alludes to the concept that blood spilt on the earth cannot be atoned except by spilled blood. Thus, Kayin was punished after killing Hevel with Galus until his own death by the hand of another. Similarly, the unintentional killer must remain in Galus until the death of the Cohen Gadol.
Shach
This verse alludes to the blood of Zecharyah. Zecharyah was assassinated in the Bais HaMikdash. Nevuzaradan (General that captured Yerushalayim) saw Zecharyah's blood boiling some 200 years later at the time of the Churban. He slaughtered the small Sanhedrin upon the blood; it did not rest. There were two small Sanhedrin in Yerushalayim of 23 judges each. This equals 46, the gematriyah of בדם. Then, he slaughtered the Great Sanhedrin of 71, but it still did not rest. כי אם is the gematriyah of 71. אשר שפך - first and last letters same gematriyah as נבוזראדן. אם בדם שופכו – first letters same gematriyah as זהו דם זכריה.
