Credit Notes & Refunds.
In HirePOS, if an invoice is returned early or overpaid, you can end up needing to provide a refund or create a credit note.
If an invoice is negative the Receive payment button will Display as Refund Payment instead, so you can simply create a refund on the spot if you wish to do so.
However, if you need to credit this negative value for use on other invoices, you may wish to create a credit note instead.