Musar sur Les Psaumes 90:2

Before G–d had begun to create the universe, He had already created the instrument called תשובה, as we know from the verse טרם הרים יולדו, "Before the mountains had been born" (Psalms 90,2). The reason is simply because G–d had been aware of how things would develop. By creating the instrument of repentance G–d created an instrument that "overrode" all things created later. This is why G–d had been able to say already to Cain: "Surely, if you do right, there is a lifting up, i.e. rehabilitation" (Genesis 4,7).