For operation reasons, we wanted to enter the "total completed till date" rather than "Quantity this period" value in the OOTB Payment Application BP.
Accordingly we modified the "quantity this period" which is now a calculated value (total completed till date - Previous Quantity). We also modified the integration interface accordingly.
However, no matter what modification we do, when we try to import the line items, we are getting an error.
"Enter a value in either Quantity this Period or Percentage Complete to Date field. You cannot enter values in both fields."
We are on 18.8.14, on premises.