We give back to God out of the abundance he's already given us.

"And he sat down opposite the treasury and watched the people putting money into the offering box. Many rich people put in large sums. And a poor widow came and put in two small copper coins, which make a penny. And he called his disciples to him and said to them, “Truly, I say to you, this poor widow has put in more than all those who are contributing to the offering box. For they all contributed out of their abundance, but she out of her poverty has put in everything she had, all she had to live on.”