Back Rated Purchases

HI Gurus,

My Client is a Automobile Based Company.

For Some of the Products the Price will be fixed based on the Market moving.(Both on Purchase and Sales)

Example.A Product will be Purchased at the fixed Price for some 5 Months.Suddenly if the Market Price of the Particular Material Falls they will get a negotiation from his Vendor like the new rate will be applicable for the previous 3 months.Called Back rated Purchases.

At that time the Amount of Purchases done on the Past 3 months has to be calculated on the new Price and

the amount of Difference has to be debited on Vendor account.

Hope this is Common in Steel Industries also

Anyone can advice me how to do this in SAP.

Tks/Rgds

Ganapathy.V