All the receipts that we have received but can't process because we can't find the event they refer to.
We hold on to them so we can process them if their event arrives later.
Remember the supplied dangling receipt.
Remove and return the dangling receipts for the given event ID.
the event ID to look for
the found dangling receipts, or undefined if we don't have one.
All the receipts that we have received but can't process because we can't find the event they refer to.
We hold on to them so we can process them if their event arrives later.