Is it too late to add a check box to the records in the sub form (they only need to be seen in the sub-form)? Then the user can check the check box, and then click the button on the main form. The main form can process all records where Check = Yes, and then reset all the checks in the table to No. So no need to worry about the wrong records still being checked from the previous transaction.
This may be somewhat problematic if this is a multi-user database, but you can process where check = Yes, and another ID field = ??
Worst case you can have the user check the record, and then put a before insert event on the sub form that takes the user's username, and then process all records where check = yes, user = username, and ID = ID.
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