Please follow the foreseen process to request corrections in SAP (raise a service ticket). For audit purposes we have to make sure such changes are documented centrally.
Following I would like to summarize my understanding of the case: You imported raw material from Baal. The invoiced amount turned out to be wrong and you received a credit note. Your problem is that you already posted the wrong invoiced amount and now have to rectify the moving average price. Do I understood correctly