Prevent quantity updating during Goods Receipt

Hi,

We have a particular requirement for a type of subcontracting process (not the standard SAP subcontracting) as mentioned below.

We create a Purchase Order for a material (without a standard cost) and giving an account assignment category in the PO. The Net price mentioned in the PO is the processing charges for the vendor (service charges). It does not indicate price for the material.

When we receive the finished material from the vendor, we perform a Goods receipt. The stock gets updated and accounting documents are created.

Our requirement is to stop this quantity updating during Goods Receipt. But, at the same time, we would like the accounting documents to be created as we need to pay the vendor.

Is there any way of achieving this?

Pl let me know. Any inputs would be appreciated.