Can you create a "Discount" item:

Item F, Qty=1, Rate=-615, Total=-615

>Dear Experts
>
>A customer goes to a shop and purchase five products (from A to E)
>
>The shopkeeper makes bill of =1815
>Now customer asks for discount of = 615
>He want to pay only 1200 instead of =1815
>
>How to adjust rates so that the bill amount will be 1200 with out any changes in quantity
>Product and Rate table is attached.
>
>Item-Qty--Rate--Amount
>A----13----60----780
>B----8-----40----320
>C----7-----35----245
>D----18----15----270
>E----25----8-----200
>
>Bill Amount= 1815
>
>Please help