PayFi Middle Layer: Engineering the “Spend” Path

Top-up & Settlement (sequence):

Top‑up → FX → Ledger → Auth → Capture → Anchor → Rewards (serialized)

1. Top‑up: User funds with MET/USDT → price source (multi‑exchange VWAP) sets base → create TopupIntent.

2. FX & net calc: Apply fees (FX/platform/fixed card fee) → compute NetUSD.

3. Double‑entry: Write into Ledger (T+0, idempotent), append audit log.

4. Authorization/Capture: Channel auth hold → capture.

5. Reconcile & anchor: Intraday reconciliation → batch Merkle root on‑chain (PaymentAnchored event) with proof fingerprints.

6. Rebates/rights: Generate rewards per merchant/quest rules (points/NFT/rebates) → write RewardLedger.

Three-phase risk:

• Pre-event: region/device/account fingerprinting, MCC allowlist, per-tx/day limits (user + rail layers).

• In-event: anomaly in frequency/amount, failure spikes, IP/device clustering → trigger KBA or temp freeze.

• Post-event: chargeback/refund workflows, merchant dispute arbitration, list updates with explainable evidence.

Why on-chain anchoring:

Anchor Merkle roots (with stats: count, net amount, region, windowId) instead of sensitive detail. This enables non-repudiation without leaking PII/merchant info. For audit/disputes, recover proofs via Merkle paths.