You can calculate interest on each invoice for the period it is outstanding, wholly or partly. Transaction by transaction or bill-by-bill interest calculation is permitted for 'Party' accounts like accounts falling under the groups Sundry Debtors and Sundry Creditors. Bill-wise details should be active for the company and the party.
The report is similar to the Bill-wise Outstandings Statement. The last column gives the interest amount on the transaction. Press F1: Detailed to see the calculation.
Go to Gateway of Tally > Display > Statements of Accounts > Interest Calculations > Ledger
Select the ledger Account XYZ Agency
Interest is re-calculated with every change in balance outstanding. The Cr in the interest amount should not be confused to mean an Income. It only means that it is payable and is a liability. The figure is illustrative only and you will need to account for the interest expense and liability through a journal entry.
To ease interest recording, Tally.ERP 9 has an interest template, a special voucher class in Debit Note and Credit Notes.