What is the procedure to adopt in case of the following situations when they arise? (FOR SAP 4.6C version) 

First situation
1) 
a) PO LINE ITEM NET PRICE = US$100 

b) The item is goods receipted with a price of US$110. Assume the difference of US$10 is because of tax caluculated by the system (for example) 

c) Vendors sends an Invoice for this item at the price of US$85 (US$80 PO line item price + US$5 tax). 

So in this case, the Invoice price is lower than the PO net price which is US$100. 
So for this type of situation what is the procedure to adopt to post the invoice in the system. 
Is it that the first the goods receipt needs to be reversed and then change or update the PO line item net price with US$80(for example) and then do the goods receipts and then do the invoice receipt? 

If this procedure is correct and what happens if vendors invoice price is lower than the origianl PO line item net price (as described in this case), whether the system blocks the invoice in this case or not? 

Or is it that the goods receipt need not be reversed and just post the invoice with the new price and change the PO price with the new vendor invocied price? 

When is that the price in the info record needs to be changed? 

Second situation 
2) What is the procedure to adopt when the vendor's invoice price is greater than PO line item price? 

Basically, I am just looking for the correct procedures to adopt in both of these cases like in what sequences,(like what is the first step and what is the second step) with what steps to follow. 

What to do procedurally - how to post an invoice with differing value than PO and/or GR and what to do if an invoice blocks, etc in both the cases like when the vendor invoice price is less than the PO price (line item net price plus tax) and one more, I need to know is in this case will the invoice gets blocked upon invoice posting when the vendor price is low. 

Similarly in the second case what to do procedurally when the invoice price is more than the PO price (Line item net price +tax)? 
 

Whatever tolerances you have set for over or under invoice will determine if any blocks occur (this covers both your first and second examples). (Transaction code: OMR6) 

When the clerk enters the invoice the system will note that the invoice is going to block but will not stop it from being posted. (Transaction: MIRO) 

Procedurally, the buyer checks blocked invoices and resolves the block be either: 

1. Changing the PO price to match the invoice and then the invoice should unblock (either automatically if you have it set up that way, or the buyer can release the block) or 

2. Buyer can dump the block without changing the PO price (info record price history is not updated in this case because PO price wasn't changed). 

In either case, if you have your materials being valuated at GR and then at IR, the invoice will revalue the material so there is not really a need to reverse the GR to change the PO price and then reenter the GR. 

The transaction code for viewing blocked invoices (and releasing them) is MRBR. 

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