There is a law of sowing and reaping as the scriptures below will show, but to be properly understood, let me say this first.
Giving should be an act of faith. For where your treasure is there your heart will be also. In other words, the thing you value most is what you spend money on the most. And the scriptures advise us to place our value in the things of God. We do this by how well we give cheerfully to the things of God.