Ok, so what I hear you saying is that you have one record in the sales table and 4 records in the rejected parts table and that is inflating your total quantity. If this is correct, here's how you get around it:
1. Group by the item field in the sales table.
2. Create a running total field:
Field to Summarize = your sales qty field
Summary = sum
Evaluate on = Formula - PreviousIsNull({sales.item_field}) or {sales.item_field} <> Previous({sales.item_field})
Reset on = change of group - the group you set up in step 1.
3. Create another running total field:
Field to Summarize = the rejected qty field
Summary = sum
Evaluate on = Every Record
Reset on = change of group - the group you set up in step 1.
4. Put your data in the footer of the group you set up in step 1. (Final running total numbers won't be available in the header or details.)
-Dell