Lock transactions when in use

At present, if someone is in a Sales Order or Purchase Order or any other transaction, and someone else goes into the same transaction, whoever Saves it last overwrites anything the other person has done.  This has resulted in Sales Orders that were dispatched being overridden and put back to Release to Pick and the order sent again.  As well as Purchase Orders that were fully receipted being changed back to parts still outstanding.  It would be extremely helpful if a transaction could be locked to prevent more than one person editing it at a time.