I propose do add a boolean field Boolean at Sales & Receivables Setup to activate or not reversing quantity shipped in case of Credit/Sales return

Yes : system will reverse : Example quantity shipped = 5, i credit back 5 -> quantity shipped goes back to 0

No : system will not reverse : Example quantity shipped = 5, i credit back 5 -> quantity shipped stays 0

Other proposition : we can add new field : quantity returned at Sales line level to avaoid confusion