What is a key disagreement between the Muʿtazilite doctrine of justice and the Sunni response described here?

The Four Proofs: Fiqh, Uṣūl al-Fiqh and Kalām

• There is no substantive disagreement concerning divine-attribute terms
• The justice debate does not involve entitlement to reward or punishment
• How to understand reward, punishment and the claim of obligation upon God
• All schools agree on one notion of external obligation imposed upon God