Interest in Payables

[bmi]

Dear Friends

I have one requirement interest invoice in Payable r12

i raised Standardard Invoice on 01 Jan 2009 and payment terms 30 days but i am making the payment 40 th day so system calculating interest for 10 over due days as per standard functionality.

but client want to pay 40 days interest for that invoice.means from invoice entered date(30 days plus over due dates 10)

i done all setups for interest invoices as per the std functionality

plese let me know possible ways

i am hoping solution asap