Matthew 5:17 is a forgery, which aside from how contradictory it is there is additional evidence in (ironically) the few surviving manuscripts of the original Hebrew. It's all elegant and poetic and full of wordplay, but at this part it suddenly changes into awful Hebrew that's clearly a translation of a Greek or Latin text, then afterward it reverts back to normal. This passage would've had to be translated back from the canonical version only if the Hebrew originally lacked it. Keep in mind that none of the other gospels have anything like it (except one line of Luke, which has already been exposed as a forgery by ancient references to Marcion's more original version).