Error - Material can no longer be changed

Hi Experts,

We are facing an error in the SRM portal. There is a PO with 4 line items where the GR/IR has been issued for all the 4. Then the user wants to increment the price of the 4th line item and so he deletes the 4th line item and adds a new line item with the incremented price. On doing this he gets a hard error stating 'Material can no longer be changed' for the line items 1 2 and 3. When he again undeletes the 4th line item and deletes the newly added line item with increased price he gets the same hard error only on line item 4.

Please help with your comments.

Regards,

Pavithra K