Below are the options that can avoid that conditions update in the condition tab of the Purchase Order : Use T-code OMET to set function authorizations and define that the conditions cannot be changed. Consider changing the condition type by using T-code M/06. If you set the indicator 'Manual Entries' to D, then it is not possible to change the condition in the purchase order.
This blog will give an overview of all standard functionalities provided by SAP from Logistics perspective which we might have overlooked during our regular practise.