I have customers invoices witch reference field (BKPF-XBLNR) is shown
in FBL5N transaction with reference Key field value (BKPF-AWKEY).
I would like to know why when I modify my Payment Method (BKPF-ZTERM)
in this customer invoice that reference field (BKPF-XBLNR) modifies
automatically showing a different value of the reference key field.
Does it mean when you change the document using transaction code FB02, it does not change but when you display the line items in FBL5N it is getting changed.
Can you tell us what is getting populated in Reference field after changing Payment method field. Probably, you can search whether any OSS notes are there for this.