John Calvin 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 for us to throw it into the treasury. Therefore, it clearly appears that hypocrites, by focusing only on the outward appearance, are guilty of treating God with gross irreverence. As long as they do not violate their Corban, (Mark 7:11), they imagine that in other matters they are pure and are unconcerned about the infamous bargain by which they, no less than Judas, had provoked God's vengeance against themselves.

But if it was unlawful to put into the sacred treasury the price of blood, why was it lawful for them to take the money out of it? For all their wealth was derived from the offerings of the temple, and from no other source did they take the money they now hesitate to return to those funds, deeming it polluted. Now, from where did the pollution come, if not from themselves?