sales order question

Hi guru:

Here is my scenario, we had placed 1000 pcs products in sales order, and we had delivered 200 pcs to our customers, then one day our customers told they don't want remaining 800 pcs anymore, what should I do in SAP, can I change order quantity from 1000 to 200 in sales order? But an error message popped up saying "item 0010 not changeable,plese contact sales order processing! m" what does it mean? What should I do?