“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.