Do you use Purchase Order for this? If you use PO:
- if you have these quantities only on order (non-posted), you only need to change these quantites
- if you have only received (non-invoiced and these quantites on stock), you can use 'Undo Receipt' for undo receiptment on order and change quantity on this order
- if you complete posted (receipt and invoice), you need to copy this document in Credit Memo and post it correction; after that you can make new Purchase Order.
Conclusion is, you can change quantity without tracking only if it is not posted; if it is posted, you need to post correction, and after that correctly entry.
If you have different case, please describe it.