PO price based on purity

hi experts,

I have raised a PO for qty of 100 nos of ROH materials type.

I have received 100 nos but the purity is not 100 % . for example 80%.

I have to pay to the vendor for 80% of purity only.

We can amend the PO price befor GR.

Kindly advice is there any other possibility in SAP

regards

s