Use the RE NXT System ID to link an existing recurring gift

When resolving an installment that must link to an existing RE NXT recurring parent gift, use that parent gift's System ID. Do not substitute its user-facing Gift ID, a GiveCampus donation/subscription ID, or the ID of an installment payment.

The validation flow retrieves the external gift by that system identifier, requires its type to be RecurringGift, and rejects an identifier already matched to a different GiveCampus recurring subscription. If the record is missing or the type is wrong, confirm the exact parent and system ID before trying again. An absent parent export must be investigated rather than hidden by creating a duplicate or skipping the installment.