If you need to amend a requisition this should be performed via the Agresso workflow process by rejecting the requisition back to the requisitioner with a comment on the requisition providing instruction for the change to be made.
This will ensure that there is an audit trail on the requisition and will prevent any glitches that can occur when an item is amended whilst in workflow by someone who is not the current task owner.
Details of this are available on our Wiki page here.