A trade was booked with the wrong quantity and you find out after it has been confirmed, reported and sent for clearing. What has to happen?

The cost of the error is a step function

Before deciding anything, establish how far the trade has travelled, because the remedy is determined by the boundaries it has already crossed rather than by the size of the mistake. Each boundary is a point after which somebody outside your firm has acted on the wrong figure.

Boundary crossed          What the correction now involves
-----------------------   ----------------------------------------------
Nothing yet               Amend the booking. Internal only.
Client confirmed          Reissue the confirmation and obtain a fresh
                          affirmation. Two firms now involved.
Reported to the           Submit a cancellation and a corrected report
regulator or the tape     under the regime's action codes, promptly.
Submitted for clearing    The clearing house holds a position that is
                          not yours. Margin was called on it.
Settlement instructed     Recall the instruction before the depository's
                          cut-off, or it will settle.
Settled                   Nothing can be reversed. Only a new trade at
                          today's price restores the position, and the
                          difference is real money.

The last row is why this question is asked. Once stock has moved against cash at the depository, there is no correcting entry that unmoves it. You are long 10,000 shares you never meant to buy, and the only way out is to sell them at whatever the market now offers. The price has moved since the original execution, so the error has produced a genuine profit or loss that belongs to somebody, and every hour of delay in noticing widens that exposure. An error caught before the confirmation costs a phone call; the same error caught after settlement costs the market move plus two sets of fees.

Amendment, cancel and rebook, and why the distinction matters

Not everything wrong requires the same instrument. The dividing line is whether the field changes the economics of the trade or only the plumbing around it.

Static and settlement data can usually be amended in place on the trade record: a settlement instruction pointing at the wrong custodian account, a missing tax indicator, a commission rate, a client reference. The trade that was agreed is still the trade that was agreed, so both sides' view of the economics is unchanged and only the downstream instruction needs reissuing.

Economic terms cannot be amended, they have to be cancelled and rebooked. Quantity, price, side, instrument and trade date all define what was agreed, so changing one is not an edit, it is the assertion that a different trade happened. The market-facing act mirrors that: a fill is corrected or cancelled by a message that references the original execution's identifier rather than by resending the original with different values, precisely so that both parties and the audit trail can see that a specific prior statement has been withdrawn. Regulatory transaction reporting works the same way, with defined action codes for cancelling a submitted report and for submitting the corrected one, and the regime expects that to happen promptly rather than in the next monthly cycle.

Side is the case worth watching, because a buy booked as a sell is a quantity error of twice the size in exposure terms. Any control that checks the magnitude of a correction should be looking at the change in exposure, not the change in the quantity field.

Never edit the row

The instinct of an engineer looking at a wrong number in a database is to correct it. In a trade store that is the expensive option, and the reason is not tidiness.

Every question anyone will later ask is a question about a past belief. What did we report to the regulator on Monday, and why did it differ from our books. What position did the risk system see when it approved the next order. What did the client's confirmation say. Which margin call was computed on which population. An updated row answers none of those, because the evidence of the earlier state has been destroyed by the fix.

So a correction is appended: a reversal of the original booking and a new booking, both carrying their own timestamps, both linked to each other and to the original, and both carrying a reason code that distinguishes a genuine amendment from an error correction. That distinction is not bureaucracy. A regulator, an auditor and a client all treat "we agreed different terms" and "we made a mistake" differently, and a store that records both as an update cannot tell them apart afterwards.

The rebooked trade keeps the original execution time and trade date. Rebooking it as of today would move the settlement date, misstate the day's activity, and make the timestamps in your best-execution and reporting records disagree with the venue's. The correction happened today; the trade happened when it happened.

The fan-out, and which parts are cheap

Once the reversal and rebooking exist, the correction has to reach everything derived from the original. Positions and profit or loss, including the prior day's if the error spans a valuation point. Risk and exposure, since the limit consumption was wrong in both directions. The margin already called on the wrong position, which the clearing house will reflect in the next cycle but which distorted today's funding. Commission, invoicing and any performance fee computed from it. The allocations to underlying funds, each of which may have struck a value on the wrong quantity. And the settlement instruction, which is the one with a hard deadline attached.

Systems built on an append-only event stream get most of that for free, because each consumer replays and arrives at the corrected state. Systems that hold mutable balances updated by messages need to be told individually, by somebody who has computed the delta, and that person is the single largest source of second errors. This is the practical argument for the immutable ledger, and it is stronger than the audit argument because it shows up on an ordinary Tuesday rather than in an inspection.

Who owns the loss, and who is allowed to sign it off

The profit or loss created by a correction does not belong to the client and must not be quietly absorbed into the desk's result, because either treatment hides an operational failure inside a trading number. Firms book it to an error account, which exists so that the cost of mistakes is visible, attributable and reportable rather than netted away. A correction that improves the firm's position is treated the same way, for the same reason: an operations process that keeps its gains and reports its losses will produce a distorted picture of its own reliability.

Approval is separated from booking. The person who made the error cannot be the person who authorises its reversal, because that combination allows a bad trade to be re-described as a good one after the fact, which is the shape of most historical trading loss cases. In practice that means a correction is a workflow with a maker, a checker and a record of both, not a privileged screen.

What a weak answer misses

The adequate answer explains how to reverse and rebook. The strong one asks what has already settled before it explains anything, because that single question decides whether this is a data exercise or a trading loss. Candidates who go straight to the mechanics of the correction have skipped the only part of the problem with money in it, and they will design a repair workflow that assumes every error is still recoverable inside the firm.