How to cancel Drop ship order line

Hi,
We have a concern regarding the Drop ship order we've made. We need to cancel the line but we encoutnered an error " You are not allowed to cancel Order Line because: Order Line has associated Purchase Orders Approved".

Here's the complete details:

Order line qty. 100 lbs
PO line qty :100 lbs
Receipt qty 30 lbs

The actual quantity received by our customer is 30lbs only so we need to cancel the 70lbs.

Can anyone help us on our concern.

Thanks!