Why does God leave giving up to the believer's free will?
God leaves giving to the believer’s free will because it displays the beauty of His grace and the Christian life. He does not mandate a set 10 percent; that amount is only a guideline. Under Paul’s teaching, no fixed amount is required. Giving is up to the believer, according as God has prospered, and God respects that free will.
Les explains that this is the beauty of the Christian life and God’s grace: He has left us with free will in giving. Believers may give as much as they want, but God does not put the thumb on their head and demand 10 percent. The 10 percent figure is a guideline for a giver, not a law; God will not zap someone who fails to give it, though they may lose reward in glory. Under Paul’s teaching there is no mandate to give a set amount. It is up to the believer, as God has prospered him, and God has enough respect for the believer’s free will that the believer will give as much as he can. Paul also wanted the Corinthians’ collection done before he arrived, so they would not give merely under his influence.
Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.