Item Cost for returned items

Hi every one,

I am new to this SAP SCN network.

Recently we implemented SAP 9 in our company,

Opening balances imported,

Now my issue is there are some items came to our inventory through sales returns,

when I invoice those items its saying that,"Item cost not found for one or more items."

what is the solution for that

further

what will be the side effect if we untick the "Allow Stock Release Without Item Cost" for that particular issue.

I am looking forward the replies ,

Thanks

VisGo