Loan Interest Calculation

Hi Experts,

Come across a new requirement from client which is as below:

Short Term Loan (product type created in money market) :  60M Start Date 1st may 2012 end date 31st Jan 2013 Interest 5%.

We have received full amount in our bank 60M, now we are paying back (installment repayment) e.g., 20M back to the Loan bank on 31st July 2012.  Here the client requirement is interest should be paid only on 20M not on 60M which suppose to be as per standard.

Please guide.

Regards,

Alok