In one familiar sense, an obligation is an act whose omission makes the agent blameworthy. For created persons, this concept may depend on a superior command, law, or assigned duty. Allah, however, is not an accountable subject under a higher ruler.
If someone says that not granting the best would render Allah deserving of punishment or blame, the question arises: what higher authority imposes this liability? The Creator–creature relation cannot simply be modeled on ordinary duties owed between created beings.
This does not deny divine justice. It rejects an externally imposed standard of liability over Allah.