Hello
Technically it is possible to change the PO Price after GR also.
Better think of Revaluation of Invoice T code MRNB
Logistics->Materials Management->Logistics Invoice Verification->Automatic Settlement->Revaluation
This T code is use in retrospective Pricing. SAP Library - Logistics Invoice Verification (MM-IV-LIV)
consider the following business case, typically where volumes are high , say automobile industry
One PO of USD 10 for 1000 Nos
Supplier is supplying material with USD10 , 500 Nos delivery is completed. Now due to forced market conditions for example rise in raw material price etc supplier is asking rate for USD12. But the case is still under negotiation with buyer. when negotiations are on , supplier can not stop the supply to company due to heavy demand, Buyer gave him assurance that after decided date (negotiated) date he will give him revised price rise , but not to stop the supply.
In such cases MRNB is used. For selected goods receipts this transaction is giving revaluation effects, means GR's in selected date ranges gets effect of USD12 rate for received quantity. Thus your inventory valuation gets corrected and it generates liability in finance.
Prerequisites for this in SAP system are
1) Vendor must be marked for revaluation allowed ( Purchasing view)
2) Purchase order shall be amended to new purchasing rate.
Thanks and Regards
Mahesh Naik