Yield Loss in Subcontracting

Dear Gurus...

We make the subcontracting PO as follows..

Requesting Mat-1 45 KG

I issue 50 KG of Mat-2

Fact is that, we have to issue 50 KG of Mat-2 and the output material (Mat-1) is only 45 KG

There is a process yied loss of 5 KG in the process.

Problem is, while we make the PO, we have to specify 45 KG for Mat-1 and Process Charge as Rs. 12...but the Vendor charges on 50 KG Qty Issued Material means payable is Rs 50x12 = 600 But system calculates Rs. 45x12 = 540 and only 540 is debitted to the Receiving material price/unit...where as we want 600 to be debitted to the receiving material price/unit...

Can anyone help solve this??

Thanks in advance

Suraj