Hi Vishal,
If Payment Terms field is not available in any data source related to customer invoice / Sales Order we can achieve this in 2 ways by using PDI.
FYI: In the customer invoice BO --> CashDiscountTerms -->Code element will holds the value of Payment Terms.
Step 1: Create a custom data source on standard Customer Invoice XBO by selecting node as CashDiscountTerms and then select require element "Code (Payment Terms element)" and in the root level select Invoice ID and UUID.
Now you have a data source which will contain Payment Terms and Invoice ID, UUID
next you can join this custom data source with standard customer Invoice data source by using Invoice ID / UUID.
Step 2: Create one extension field, in the before save event copy the value from "CashDiscountTerms -->Code" to the extension field, after enhance this newly created extension field to datasource.
In this case you will not get old data, this will be applicable for only new / updated records.
I hope it will clear your requirement.
Regards,
Shiva G