Payment Systems and the Cash Leg

Every securities trade has a cash leg, and that cash doesn't move by magic — it moves through wholesale payment systems, the plumbing that transfers money between banks' accounts at the central bank. Understanding this layer matters because it has its own rules, timing, and risks, separate from whatever happens on the securities side of the trade.

The most important distinction is between real-time gross settlement (RTGS) systems and net settlement systems. In an RTGS system — Fedwire in the US, CHAPS in the UK, TARGET2 in the eurozone — each payment settles individually and immediately in central bank money, one transaction at a time, and once settled it is final and irrevocable: the receiving bank can rely on that money being genuinely theirs with no risk of it being reversed later. In a net settlement system, payments accumulate throughout the day and only the net difference between banks is actually transferred at specific settlement times, which is cheaper and needs less liquidity moving around, but means individual payments aren't final the instant they're sent — they're provisional until the whole day's net settlement completes.

That gap between "sent" and "genuinely final" is where payment-system risk lives. If a bank in a net settlement system fails between sending a payment and the final settlement moment, the payment it sent might be unwound, leaving the receiving bank short of money it thought it had. This is a close cousin of the Herstatt risk problem in delivery-versus-payment — the risk of relying on a leg of a transaction before it is truly, irrevocably done — and it's exactly why the largest, most systemically important payment flows have migrated toward RTGS systems over recent decades, trading some operational cost for a hard guarantee of finality.

Cross-border payments add a further wrinkle: a payment in a foreign currency typically has to pass through that currency's own domestic payment system to actually settle, and different countries' systems keep different operating hours and cut-off times. A dollar payment initiated late in the European trading day can miss the Fedwire operating window and not actually settle until the next US business day — a timing gap that traders and treasury desks need to plan around explicitly, since "the trade settled" and "the cash has actually, finally arrived" aren't always the same moment when currencies and time zones cross.