A credit invoice can be created similarly to a purchase invoice and a sales invoice.

Let’s create a sales invoice.

1. Open “OPERATIONS” and select “Invoices”.

2. Find the invoice you want to credit. Open the relevant invoice by clicking on the invoice line.

3. From the Operations menu on the right select “Create credit invoice”.

4. The system enters all the necessary details automatically. Check for correctness.
For partial crediting change the amount to be credited manually.
Save by clicking on “Save invoice”.
N.B. If the invoice is declared in a previous month, the date of the credit invoice must be a date of the current period, VAT return not yet filed.

P.S. The manual on how to offset a sales invoice against its credit invoice can be found here:
Marking credit invoice as paid.