Net Price in PO ME21N cannot edit if Down Payment already created

Hi all SAP Experts,

Based on the topic, where i want setting in ME21N (create PO) expecially for Net Price cannot edit if Down Payment already created. But honestly, my question is i want to setting Net Price cannot edit lower than the Down Payment Amount already created.

Let me explain more detail with example :

First - ME21N - create PO

Qty = 10

Net Price = 1000

Net Value (PO) = 10.000

DP Category : M - Mandatory Down Payment

Down Payment % = 60%

Down Payment Amount = 6.000

Save PO and Release PO.

Second - ME2DP (Down Payment Monitoring for PO)

Third - back to ME21N after un-release PO, then go to ME22N to edit Net Price from 1000 become 500.

So, in this position then u can see the total PO Amount now is Qty * Net Price = 10 * 500 = 5.000 which this amount is lower than Down Payment where already pay to vendor.

Finance Div, Accounting Div and Purchasing Div in our company want to block Net Price in PO cannot edit lower than total PO.

Oh..one more question is why SAP allowed for net price in PO still can editable eventhough invoice (MIRO) alreay created for the PO that actually if i can see in Purchase Order History (POH) in PO, shown incorrect data if mapping with total PO Amount with total MIRO Amount (because last editable).

Please, anyone can sharing for this issue. Really appreciate for any reply.

Thanks.

Haidi