Charles Ellicott Commentary Matthew 27:6

"And the chief priests took the pieces of silver, and said, It is not lawful to put them into the treasury, since it is the price of blood." — Matthew 27:6 (ASV)

It is not lawful to put them into the treasury — The Greek for the last word is the Corban, or sacred treasure-chest of the Temple, into which no foreign coins were admitted, and from which the Law (Deuteronomy 23:18) excluded the unclean offerings of the price of shame, which featured largely in the rituals of many pagan nations. By parity of reasoning, the priests seem to have thought that the blood-money which was returned was also excluded.