Jesus is using a Euphemism that all his listeners understood, but which we don't have in English, and so is very misunderstood today. To have a good eye was a euphemism for being generous, while having a bad eye was a euphemism for being stingy. What he is saying is that if you are generous with your money, you will be generous with your time and other possessions as well, but if you are stingy with your money, you will be stingy in other areas as well.
If you look at the context, you'll see that he just finished talking about money and material possessions, and follows this up by talking about money and material possessions.
For where your treasure is, there your heart will be also. (Matt. 6:21 ESV)
No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other. You cannot serve God and money. (Matt. 6:24 ESV)
Why would he start by talking about money, then say a couple of sentences about something entirely different, then start talking about money again? It all makes more sense if we understand that he was talking about money the whole time.
The TOG