Usually, green card application costs you around $5000 if you retain an attorney. Most attorney accept credit cards. There if you can pay your attorney fee with a up to 5% cash back credit card, you will save $250. Even with a up to 2% cash back credit card, you can save $100. Although it is not a lot of money, but you can still buy some stuff with it. The application is quick and easy.
Online Credit Cards Center is a very useful website, which give you a lot of options for cash back credit cards, rewards credit cards, etc. You can even pay your tuition with your cash back credit cards.