In some cases, you may want to have an item that you use to provide a special type of refund for a customer. An example would be if you needed to pay a customer for a winning lottery ticket, or refund a despot, a manual adjustment to an account, etc.
If the amount is known then you can create an item and return it (creating a negative amount) or you might want to create an item with a negative value.
If the amount is not known, for example, with a lottery paid out then you can create a "prompt for price" style item in the back office and if you set its retail price to negative $1, then anything you input when it prompts you for a price will have a negative value. So, if the cashier inputted $5.00 it would show at the POS as a negative $5 item.
As with any situation where money is coming out of your till, you need to be reviewing your end of day reporting and reconciling these events to make sure they are legitimate.