Ramban, on Gen 4:7

“If thou do well, shalt thou not receive? but if ill, shall not sin forthwith be present at the door? but the lust thereof shall be under thee, and thou shalt have dominion over it.”

“IS IT NOT THUS, IF THOU MENDEST ‘SE’EITH’. In the opinion of the commentators, this means there is a “lifting” or forgiveness of your sin. And in the opinion of Rabbi Abraham ibn Ezra it means a “lifting” of your face in contrast to [the question G-d asked of Cain]: Why is thy face fallen? For he who is ashamed presses his face downward. Similarly, it is said, And the light of my countenance they cast not down, whereas one who honors him is as if he raises his face upward. This is the sense of the verses: Perhaps he will lift my face; Do not lift the face of the poor. In my opinion the verse means: “If you will mend your ways you will have your rightful superiority in se’eith (dignity) over your brother since you are the firstborn.” And this is the meaning of [G-d’s question to him]: Why art thou wroth? For by virtue of his feeling ashamed before his brother, his face fell, and because of his jealousy of him he killed him, and now the Eternal told him: Why art thou wroth regarding your brother, and why is thy face fallen on account of him? Is it not thus! If thou mendest, you will have superiority in dignity over your brother, and if thou dost not mend, evil will come upon you not only because of him [your brother], for at the door of your house your sin lurks causing you to stumble in all your endeavors. AND UNTO THEE IS ITS LONGING, for your sin longs to cleave to you at all times. Nevertheless thou mayest rule over it if you so desire, for you may mend your ways and remove it from upon you. Thus He taught him [Cain] concerning repentance, that it lies within his power to return anytime he desires and He will forgive him.”

Imported from an open dataset — not yet checked against the printed edition.