Credit memos that do not have a value date will be immediately due for payment on the billing date. This means that the receivables and payables will not be reconciled.
When defining billing types in Customizing, you can select the 'set value date for a credit memo' function.
If this function has been selected in the billing type, the reference billing date has not been cleared, and the baseline date for payment in the base billing document is after the billing date in the credit memo, then field:
VALDT (fixed value date) is filled with the baseline date for payment from the base billing document, and the field
REBZG (document number) is filled with the FI document number.
If the base billing document has been reconciled and/or the baseline payment date is before the billing date in the credit memo, then the FI document number is copied into the REBZG field.
You can switch on the value date setting for a billing type in the "Credit memo with value date" field. "Define billing types".