The Justice of Allah (SWT) is The Absolute Justice which has nothing but Justice and fairness. Injustice comes from people who need something and try getting it in a wrong way of injustice. Allah (SWT) Has no need at all, so, there is question of injustice from Allah (SWT).
Different opportunities is part of the Justice of Allah (SWT) as every one will face his results according to his own opportunities.
Wassalam.