On Odoo 8, we were able to add lines on an invoice which had negative amounts, typically used to credit deposits (recorded to the balance sheet) previously paid so that correct amount is now due.  

Please tell me how to enable this on Odoo v15.  It will be cumbersome to the extreme if we have to raise separate credit notes and issue new invoice at full value, apply the credit to the invoice and then explain to the customer that although we invoiced full amount to only pay the difference