IF you are unable to save the invoice you created, it may be the reason is you are login as sub user,and its user restrictions,To give rights you can follow given steps:
Step 1:Go to the Administrtaion menu,
Step 2:Click on users,
Step 3:Open that user in modify mode,click menu rights, here you can give rights to this user,click on transactions
Step 4:check the check box sales, then check modify,to give rights,
then re open the company as sub user again then you can save the invoice again and modify