Discharging a prior debt is not the same as freely bestowing an additional favor. Guidance, sustenance, and relief are understood as Allah's generosity and favor, not as the settlement of an externally imposed debt.
Imtinān means bestowing a favor for which gratitude is due. If every beneficial gift already constituted an enforceable entitlement of the servant, it would lose the very character of discretionary grace at issue here.
This does not diminish divine power or mercy. It distinguishes a purported rationally imposed obligation from freely granted beneficence.