What ought to be done in the event a trade erroneously occurs at an off-market...

By agreement between the two counterparties, the trade must be cancelled as soon as practically possible since a rate amendment is prohibited.

By agreement between the two counterparts, the trade should, as soon as practically possible, either be cancelled or have its rate amended to an appropriate market rate.

The off-market rate should be adjusted as soon as possible to the appropriate current market rate and a new authenticated SWIFT confirmation sent immediately to the counterparty.

Nothing need be done, since once a trade is agreed to by the front office it is a binding agreement for both counterparties.