Is the disparity between rich and poor a result of the rich's earning, or of God's decree as a trial, or due to His knowledge of what is best for their condition, and does the verse: (We have apportioned among them their livelihood in the life of this world and raised some of them above others in degrees) suggest predetermination in sustenance?

Allah has divided and destined sustenance among people, and He has made a cause for everything. He has thus prescribed effort and earning, while seeking His help. And the strong are more beloved to Allah than the weak. A servant is given according to his ambition and intention. The disparity in sustenance among people is due to profound wisdom, including the diligence and skill of some, and the laziness of others, as well as the assignment of some servants to specific professions, and also includes piety and avoidance of disobedience. Poverty might be better for some than wealth, and vice versa. Among these reasons is the trial of the rich in their gratitude, and the trial of the poor in their patience and contentment. Wealth does not indicate honor, nor does poverty indicate humiliation.