Late Charges for Partial Paid Invoice

Dear All,

System is not calculating correct interest on Partial paid Invoice

For Example:

Invoice Date 01-Jan-2015 and amount 1000

Immediate Payment Term

Receipt applied on 25-Jan-2015 of 500.

When i run late charges program on 31-Jan-2015. System is calculating late charges for outstanding balance (500 amount for all 30 days) but our requirement that system calculate interest 24 days for 1000 amount and 7 days on 500 amount.

Thanks