OptionalamountOptionalcurrencyISO 4217, when the payload reports it alongside amount.
Stable dedupe key. The consuming application owns the "have I already processed this event id" store — PayFanout persists nothing.
ISO 8601. Webhook delivery is unordered on every PSP — treat events as unordered facts.
OptionalpspOptionalrefundThe PSP refund id, on refund-shaped events.
Money facts, normalized where the PSP payload carries them — a stateless host should not need a retrievePayment round-trip to know how much a payment.refunded event refunded. Integer minor units.