From The Secret - fair exchange

Incorporating the rule of fair exchange in your life is living the law of attraction.
Be sure that you always give fair exchange in business and in your personal life. In business, always give more value than the money you receive. In your personal life, always give back what you have received. If you have received support from others at a time when you needed it, and then someone asks for your support, be sure to give back the support you have received. All support comes to you through the Universe, and so when you give support back you are giving back to the Universe.
This is living according to the rule of fair exchange, and it is living the law of attraction.