In most cases we recommend you use the 'Credit Memo' function when issuing a customer a refund or credit to ensure proper inventory tracking. However, if you need to add a credit as a line item, here's how:
Create an item in ZarMoney named 'Credit' using the item type 'Discount Item.'
This item will map to your Profit & Loss as 'Sales Discount.'
When you create an invoice, add the item 'Credit' as a line. This will default as a negative amount directly on your invoice.
Then click 'Save & Close' to finish