Blocking Price Change in PO

I need to know how to block the price in a Purchase Order from being change on initial entry when the PO is with reference to an Outline Agreement (Contract) without blocking the ability to change the price for other types
of POs being entered. I have tried using Transaction OMF4 in configuration. But if I change for document type ME21 the Price and price unit to display only it changes it for all POs not just those that are created from contracts.

I think that the Invoice Verification component is part of the Materials Management (MM) system. It provides the link between the MM component and the Financial Accounting, Controlling, and Asset Accounting components.

Invoice Verification in Materials Management serves the following purposes:
- It completes the materials procurement process - which starts with the purchase requisition, continues with purchasing and Goods receipt and ends with the invoice receipt.
- It allows invoices that do not originate in materials procurement (for example, services, expenses, course costs, etc.) to be processed.
- It allows credit memos to be processed, either as invoice cancellations or discounts
Invoice Verification does not handle the payment or the analysis of invoices. The information required for these processes is passed on to other departments.
Invoice Verification tasks include:
- Entering invoices and credit memos that have been received
- Checking the accuracy of invoices with respect to contents, prices, and arithmetic
- Executing the account postings resulting from an invoice
- Updating certain data in the SAP system, for example, open items and material prices
- Checking invoices that were blocked because they varied too greatly from the purchase order
An example on how to enter an Invoice Receipts :-
Transaction code MRHR - MM Invoice Verification and Material Valuation

There are two ways to force close a PO:
1. You can set Delivery Completed Indicator in Purchase Order or at time of doing goods receipt. That simply means you cannot do further GR for that item. You can still process Open Invoices .
2. You can block material at purchase order level which means, you will not be able to do any GR , Invoice against that specific item in PO.
The recommended is the first option though.

Second option is used in case of : -
1. to cancel an item
2. An item is closed and can be archived

MEI1: Automatic purchasing document change.
MEI6: Delete purchasing document index.
ME52N: Change purchase REq.