With the template composer, it's easy to create a credit invoice, with the same template as the one you use for regular invoices.
Learn here how to generate a credit invoice.
Multiplier
{% assign multi = 1 %}{% if invoice.type == 'credit' %}{% assign multi = -1 %}{% endif %}
The code above creates 'multi'. We will use this to multiply with either 1 - when it's a regular invoice, or with minus 1 - when it's a credit invoice. The system will recognize what to use when.
The first step is to add this code to your template, preferably at the top, so it will effect your whole template.
The second step is to add the multiplier "multi" to your amounts.
You can add it on any amount. For example like this:
{% event.fee | times: multi %}
The system will know when to multiply your event.fee times 1 or -1.
When adding this, it might be useful to have a test event and test invoice ready. Check out how to do that here!
If you need any help, please contact Support.