How to add Fixed Principal payment type at payment account boarding:

Please review KB article How to Board a Payment Account in Payments. 

How to add Fixed Principal payment type as a future transaction change:

Please review KB article Schedule Future Transactions: Irregular payment schedules and Seasonal loans.

How Fixed Principal works in Payments:

After setting the fixed principal amount, either through payment account boarding or scheduling a future transaction change, the system will calculate the total payment amount every month at statement generation. During this process, you will see a Set Payment Amount/Type transaction type recorded in the Transactions menu before your generated statement to set the new payment amount. This calculation will be the set fixed principal amount plus the accrued interest due.