Price to be unchanged in Retuns sales order

Dear All,

My client's req is like this...OR >LF >F2-->RE (returns Sales order)

If I create Returns Sales order-RE with reference to Billing doc-F2, all the relevant feilds get copied into Returns sales order including pricing elements.Here we can change the pricing elements manually at item level.But my req is once the price get copied into Returns sales order, it should not be changed( PR00 value should be uneditable/grey mode).

Points assured

Krishna Chaitanya