Gave them for the potter's field. This part of the citation is borrowed from Jeremiah's purchase of the field of Hanamel (Jeremiah 32:1-44.). The Christian writer introduces a second fulfilment of the ancient word.
As the Lord appointed me. This must be the equivalent of Zechariah's "the Lord said unto me" (Zechariah 11:13). The destination of these wages of iniquity was foreordained. They could not be used by the Shepherd, nor stored in the temple treasury, nor kept by Judas or the priests; they were to be employed for another purpose.