hi,
Please check the below link, might be helpful.
Return Purchase Order-Negative delivery costs n... | SCN
I am no MM expert, but following option can be checked. If the qty received is less than the qty in the PO and no further receipts are expected. you can check the check box 'Delivery completed'. this will show the PO as closed and also not show up the additional qty as commitments.
Please check and let know if it works.
Ideally creating another set of PO's to negate the commitment is additional burden to the process and we should try to reduce the number of steps just to make the process easy.
thanks