If the capital sum field isn't editable when you set up a reduced balance type deduction, this is because the reducing balance indicator isn't enabled at pay code level. To enable this indicator, follow the steps below in back office.
Click CorePay then click Reference.
Click Pay Codes.
In the Pay Code field, to select a pay code, click the list
icon.
In the Deduction tab, enable the Reducing Balance check box.
Click the save
icon.