The phrase "you have heard" does not refer to the scripture, it refers to oral teachings of the Pharisees who violated scripture with their own tradition, as per Matt 15.3:

But he [Jesus] answered and said unto them, Why do ye also transgress the commandment of God by your tradition?"

Cursing enemies was a regular part of that Pharisaic tradition as well as traditional teachings in other sects.

• In the Amida prayer discovered in the genizah of the Ben Ezra Synagogue in Cairo[1], an entire section was devoted to cursing enemies. This prayer was said three times each day, four times on the Sabbath, and five times on High Holy days. Here, they curse Christians (Natzarim), The Roman Empire, and what are believed to be Sadducees as enemies:

For the apostates let there be no hope,

למשומדים אל תהי תקוה

and may the kingdom of the arrogant

ומלכות זדון

be quickly uprooted in our days;

מהרה תעקר בימינו

and may the natzarim and minim

והנצרים והמינים

instantly perish;

כרגע יאבדו

may they be blotted from the book of the living,

ימחו מספר החיים

and may they not be written with the righteous.

ועם צדיקים אל יכתבו

Blessed are you Lord,

ברוך אתה יי

humbler of the arrogant.

מכניע זדים

• The Qumram communities required hatred of their enemies as part of the rules of the order[2]:

(4) that he has chosen and hate all that he has rejected; that they may abstain from all evil [...]

(10) according to his lot in God’s design, and hate all the Sons of Darkness, each according to his guilt (11) in God’s vengeance. All those who freely devote themselves to his truth shall bring all their knowledge, powers,

(21) from all injustice. These are the rules of conduct for the Maskil in those times with respect to his loving and hating everlasting hatred (22) toward the men of the pit in a spirit of secrecy. He shall leave to them wealth and earnings like a slave to his lord and like a poor man to

Part of the reason for this may have been the tradition of both blessing and curses on mount Ebal/Gerizim (Deut 11.26), but whatever the historical reason, to a pharisee curses and blessings were the flip side of the same coin. They blessed their friends and cursed their enemies in the same breath and as part of the same prayer.

1. David Instone-Brewer, Prayer and Agriculture, vol. 1, Traditions of the Rabbis from the Era of the New Testament (Grand Rapids, MI; Cambridge, UK: William B. Eerdmans Publishing Company, 2004), 97–101

2. Alex P. Jassen, “Rule of the Community,” in Outside the Bible: Ancient Jewish Writings Related to Scripture: Translation, ed. Louis H. Feldman, James L. Kugel, and Lawrence H. Schiffman, vol. 3 (Philadelphia: The Jewish Publication Society, 2013), 2929.

There is not any meaningful or any significant difference for in both cases the same reality of God's word is referred to. The Holy Scripture was customarily read aloud in synagogues, thus "you have heard" is matching to "it is written".

Thus, Jesus does not refer to Pharisees, Sadducees, or any Jewish theologians' interpretations of the Holy Scriptures when saying "you have heard", but to the Scriptures themselves. The crucial point here is another thing: Jews noticed that Jesus, in difference from Scribes and Pharisees spoke "as a one having authority" (Matthew 7:29). But what does it mean to "have authority"? Why it was so drastically different from the Scribes and Pharisees? Did He speak with a louder voice or with more sparkling eyes displaying a greater inspiration? Not, of course! The reference is not to how rhetorically impressively He spoke but the contents of His speech, the "what" of His speech.

Now, this contents is that He did not take a passage, a quote from Holy Scripture and then interpret it like the Scribes and Pharisees, but changing at will the very passage! Saying that henceforth not the previous passage should be quoted by theologians, but His novel words! That is to say, He substitutes the Scriptural passage with His words, giving a new Scripture. Exactly that is the essence of His scandal, that is the essence of people's bewilderment that "He is speaking as one who has authority to change the very Scripture and establish the new Scripture"! It is the same as when He authoritatively says: "I give you a New Testament" (Matthew 26:28), thus saying that He is either equal to or identical with the one who gave the Old Testament - God. For, in fact, only the one who has the equal and the same authority as God can change at will the words of God.

To give an analogy: if a Roman Emperor legislates: "Roman citizenship should be given only to the inhabitants of Latium and not to anybody else". Then, with changed circumstances in the world, when another Roman Emperor sees that now it will be profitable both for Empire's well-being to give citizenship to others as well, he overrules at will the previous legislation with a new one: "Roman citizenship should be given to all, who will provide a worthy service to it regardless nationality". But only Emperor can overrule an edict of an Emperor. Anybody else, who would do so, would be a criminal. The same here: unless Jesus is God, then overruling the word of God is nothing but a blasphemy. Therefore, the passage gives a clear indication of Jesus' claiming for Himself a divine authority.

One may ask, "but does God oppose God?", for Jesus clearly overrules the Scriptural precepts. Not of course, but God apportions commandments with a consideration of the condition of the listeners' abilities, measuring the commandment as optimal to this condition. In theology this divine consideration, measurement and apportioning is called "economy" (οἰκονομία). Thus, "eye for eye, tooth for tooth" is God's commandment, but the same God made it obsolete after coming to earth in human flesh to teach people the perfect gospel, that they may become as perfect as their Heavenly Father who has sent Him to them.