If the final total is +ve (or zero) the regular Sale Checkout Flow is performed; if -ve, the Return Checkout Flow is performed. POS Settings - Saved Searches:
Behind the Scenes
Sale Checkout Flow: A invoice is created and a payment is applied directly to the invoice; in addition a credit memo is created and applied to that invoice.
Return Checkout Flow: A credit memo is created and a refund is applied directly to the credit memo; in addition a invoice is created and applied to the credit memo.