When creating credits directly from the invoice tab of a purchase order, it will block the creation of the credit if the value of the credit is below the minimum value set in the order management settings of the supplier.
Click the Admin Cogs along the top.
Click Suppliers, then click Manage Suppliers.
Locate the supplier and click the Magnifying Glass.
Click the Order Management tab.
Locate the section titled credit and return settings.
Locate the setting minimum net credit value and enter a numerical value.
Click Save Settings to confirm any changes.