What does the bad eye refer to in Matthew 20:15? It refers to an eye that cannot see the beauty of grace. It cannot see the brightness of generosity. It cannot see unexpected blessing to others as a precious treasure. It is an eye that is blind to what is truly beautiful and bright and precious and God-like. It is a worldly eye. It sees money and material reward as more to be desired than a beautiful display of free, gracious, God-like generosity.
That is exactly what the bad eye means in chapter six of the Sermon on the Mount. And that meaning gives Matthew 6:22-23 a perfect fitness between a saying on true treasure (Matthew 6:19-21) and the necessity of choosing between the mastery of God and the mastery of money (Matthew 6:24).
So the flow of thought would go like this: Don't lay up treasures on earth, but lay up treasures in heaven. Show that your heart is fixed on the value that God is for you in Christ. Make sure that your eye is good not bad. That is, make sure that you see heavenly treasure as infinitely more precious than earthly material treasure. When your eye sees things this way, you are full of light. And if you don't see things this way, even the light you think you see (the glitz and flash and skin and muscle of this world) is all darkness. You are sleepwalking through life. You are serving money as a slave without even knowing it, because it has lulled you to sleep. Far better is to be swayed by the truth—the infinite value of God.
So if you are emotionally drawn more by material things than by Christ, pray that God would give you a good eye and awaken you from the blindness of "the bad eye."