How to return Balance in Carry Forwarded Budget after Purchase Order created

Hi Experts

My client requirement is not to make balance of carry forwarded budget  available in current year. It should return to previous year.

Suppose a PR is made in 2013 of amount  1000000.00 for 10000 qty.

value per qty is 100.00.

In the year 2013 PO created for  5000 qty is of amount 500000.00.

And at year end  remaining amount 500000.00 has been carry forwarded to 2014.

Suppose now material price is reduced in 2014 to  80.00 per qty.

Now in 2014  PO created  for remaining 5000 qty and amount 400000.00 will be consumed and balance amount 100000.00 will be freely available in year 2014.

As per requirement the balance amount 100000.00 should not be available for 2014. It should return to 2013.

Please suggest me how to achieve this functionality.

Its showing available in 2014 in report 'FMRP_RW_BUDCON'.

Is there any standard configuration available for this or need make derivation rule of any other way is there.

Waiting for your reply.

Thanks

Ranjan.