Balance Interest Calculation on Both case (Advance & Invoice)

Dear Guru,

I have a doubt in Balance Interest calculation:

My client required Balance interest calculation on advance & Invoice both case before clearing advance.

For example.

Interest Rate is @13.50%

Advance amount received from Customer Rs. 7000/- on 01.11.2014

and book a Invoice on 10.11.2014 Rs. 17000/-

Balance amount = (Rs.17000-Rs.7000) = Rs. 10000/-

Client required interest Calculation scenario as under

Rs. 7000.00 X 13.5% X 9/365 =  Rs. 23

Rs. 10000.00 X13.5% X21/365 = Rs. 78

So Interest credit Rs. (78-23) = Rs.55

So closing balance of Customer = (Rs. 10000 + Rs. 55) = Rs. 10055.00

Kindly provide all possible solution for above scenario.