How to Restrict PO Line Reprice on Quantity Change ?

Hi,

We have Quotation to PO Process.

Buyer changed some po line prices which created from the Quotation.

Example : For Item A Quotation price is 100

But after creating the PO from the Quotation, buyer changed the price to 90 (Supplier provided special discount for that PO).

So in that case during the Quantity/ need by date change, Po line price will be changed to Quotation price.

My question is : How I can restrict the PO line reprice when modifying the Quantity or need by date.

Thanks,

Lingaraj