How can You say that extra money earned by a person should be returned to God since it is God's property?

[Money is earned by the efforts of the person who worked for it and hence, it must belong to him.] It is not correct to say that the extra money comes by extra effort. You can find several people, who do not get even the basic needs in spite of hectic efforts. Therefore, whether you get the basic needs or extra money, it is by the grace of God alone. Hence, you are expected to donate something to God, which shall be used in the work of God.