Detail line editable for nonstock item

Hi,

I am placing the purchase order for non-stock item where at detail level after entering the item record in item number field press enter to move to next line but haven't save the order, suddenly I realize that I have enter the wrong item number and tried to change that but not able to do that.

I have checked the same thing with stock item, there system allows me to change the item number.

So why I am not able to do the same thing for non-stock item and how I could enable this for non-stock item.

What setup required to fulfill this requirement.