"Zacchaeus stood there and said to the Lord, `Look, half of my possessions, Lord, I will give to the poor; and if I have defrauded anyone of anything, I will pay back four times as much.' Then Jesus said to him, `Today salvation has come to this house, because he too is a son of Abraham. For the Son of Man came to seek out and to save the lost.'" - Luke 19: 8-10 (NRSV) If we merely give out of our excess we don't give God the opportunity to prove his faithfulness. That's like lifting a very light weight. It won't strengthen your muscles. But when we give sacrificially, when we give to the point of it hurting a bit, we exercise our faith - we learn to trust in God by giving him a chance to prove himself trustworthy.
As he looked up, Jesus saw the rich putting their gifts into the temple treasury. He also saw a poor widow put in two very small copper coins. "I tell you the truth," he said, "this poor widow has put in more than all the others. All these people gave their gifts out of their wealth; but she out of her poverty put in all she had to live on." â€“ Luke 21:1-4 Far too many Christians imagine that they'll be more generous if and when they become rich, but the amount of money we have doesn't matter. Generosity has more to do with the proportion we give than it does with the actual amount we give...no matter how much we actually own
"Each one should give what he has decided in his heart to give, not reluctantly or under compulsion, for God loves a cheerful giver." - 2 Corinthians 9:7 People give for lots of different reasons, but not all giving accomplishes what God wants to accomplish in our lives. We don't give for God's sake; we give because we need to...we give because God wants to do something in our lives through our giving.