It is not possible to update the service rendered date in the customer project sales orders / billing documents.
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
SAP ERP Project System (PS)
- SAP S/4HANA, cloud edition
Reproducing the Issue
Create Debit memo request (DMR) by editing customer project, with keeping Service date blank.
- Open the Debit memo request, service rendered date is filled and cannot be changed.
This is designed behavior.
When you use the system automatic to create billing proposals which afterwards can be billed, the billing proposals will use the period (month) end date as relevant date.
VBKD-FBUDA, FBUDA, billing date , KBA , PS-FIO-REV , Fiori UI for Project Systems-Revenues and Reso. Rel. Billing , PS-FIO , Fiori UI for Project System , Problem