Why can't I edit an invoice?

You can edit an invoice up until it has had a payment or a part payment allocated against it - at this point it becomes locked. In the screenshots below, the invoices highlighted have received a part payment (i.e. their outstanding balance is smaller than their total balance). Therefore they can not be edited at present. This results in some fields being 'greyed-out' when you open the invoice.

'Unmatch' to edit:

If you want to edit an invoice that has reached this stage, you will need to first go to the bank statement screen in 'Bank Reconciliation', and click the 'unmatch' button to the right of the income transaction(s) that have been allocated against the invoice. You will then be able to edit the invoice, and then re-match the payment against it in the bank matching screen.

To help you better understand the invoicing module as a whole, please visit the module overview page here.